Method for producing photovoltaic device

ABSTRACT

A photovoltaic device comprises a metal with a smooth surface; a transparent conductive layer formed on the smooth surface; and a photoelectric conversion layer formed on the transparent conductive layer. The transparent conductive layer has an irregular surface at a side opposite to the smooth surface.

This application is a division of application Ser. No. 08/341,948 filedNov. 16, 1994, now U.S. Pat. No. 5,500,055, which in turn is acontinuation of application Ser. No. 08/013,109, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photovoltaic device of a highconversion efficiency, high reliability and satisfactory massproducibility, and a method for producing the same, and it also relatesto a secondary battery, an artificial satellite, a roofing material, asolar cell module, an automobile or the like, utilizing a solar cell asan example of said photovoltaic device.

2. Related Background Art

For most future energy sources, there are involved various problems,e.g. in the total reliance on fossil fuels such as petroleum and coal,the use of which is considered to result in warming of the earth becauseof the by-product carbon dioxide etc., and on nuclear power, whichcannot be freed from the danger of radioactivity in case of anunpredictable accident or even in ordinary operation. In contrast, solarcells utilizing solar light as an energy source have very littleinfluence on the environment, and are expected to be used for manyapplications. At present, however, there are some drawbacks that hinderpractical use of such solar cells.

For solar power generation, mostly monocrystalline or polycrystallinesilicon have been utilized. However, such solar cells, as an example ofthe known photovoltaic cells, require a large amount of energy and timefor crystal growth and also require complex subsequent steps, so thatmass production is difficult to achieve and supply thereof at a lowprice has therefore been difficult. On the other hand, there have beenconducted active research and development on so-called thin filmsemiconductor photovoltaic devices utilizing amorphous silicon(hereinafter represented as a-Si), or compound semiconductors such asCdS or CuInSe₂. Such photovoltaic devices can be obtained by forming thenecessary semiconductor layers on an inexpensive substrate such as glassor stainless steel with a relatively simple process, and therefore offerthe possibility of cost reduction. However, such thin film semiconductorphotovoltaic devices have not been employed in practice because theirconversion efficiency of light is lower than that of the crystallinesilicon photovoltaic devices, and also their reliability under prolongeduse has been low. For this reason, there have been made various attemptsto improve the performance of the thin film semiconductor photovoltaicdevices.

One attempt consists of providing a rear reflective layer for returningthe light not absorbed by the semiconductor layer to said semiconductorlayer, in order to achieve effective utilization of the incident light.For this purpose, in the case where the light is introduced through atransparent substrate, the electrode formed on the surface of thesemiconductor layer is composed of a highly reflective metal such assilver (Ag), aluminum (Al), or copper (Cu). Also, in the case where thelight is introduced from the top side of the semiconductor layer, asimilar metal layer for increasing the reflectance is formed on thesubstrate, before the formation of said semiconductor layer. Also, atransparent layer with a suitable optical property may be providedbetween the metal layer and the semiconductor layer, for furtherimproving the reflectance by multiple interference. FIG. 4A shows thereflectance in the case where such transparent layer is not presentbetween silicon and various metals, and FIG. 4B shows the simulatedresults of improvement in reflectance, in the case when a zinc oxide(ZnO) layer is provided as such transparent layer between silicon andvarious metals.

Such a transparent layer is also effective in improving the reliabilityof the photovoltaic device. Japanese Patent Publication No. 60-41878discloses that such a transparent layer prevents alloy formation betweenthe semiconductor and the metal layer. Also, U.S. Pat. Nos. 4,532,372and 4,598,306 disclose that a transparent layer with a suitableelectrical resistance prevents the generation of excessive currentbetween the electrodes even in the case of short-circuiting in thesemiconductor layer.

Another attempt to improve the conversion efficiency of the photovoltaicdevice consists of employing a texture, having fine irregularities, onthe surface of the photovoltaic device and/or the interface with therear reflective layer. In such configuration, the light is scattered onthe surface of the photovoltaic device and/or at the interface with therear reflective layer and is confined in the semiconductor(phototrapping effect), whereby it is effectively absorbed therein. Inthe case where the substrate is transparent, the texture is formed onthe surface of a transparent electrode, for example, of tin oxide (SnO₂)formed on the substrate. Also, in the case where the light is introducedfrom the top side of the semiconductor, the texture is formed on thesurface of the metal layer employed as the rear reflective layer. M.Harasaka, K. Suzuki, K. Nakatani, M. Asano, M. Yano, and H. Okaniwareported that an irregular texture for a rear reflective layer could beobtained by Al deposition under control of the substrate temperature andthe deposition rate (Solar Cell Materials, vol. 20 (1990), pp. 99-110).FIG. 5 shows an example of the increase of absorption of the incidentlight, through the use of a rear reflective layer with such texture,wherein curve (a) indicates the spectral sensitivity of an a-Siphotovoltaic device employing smooth Ag as the metal layer, while curve(b) indicates the spectral sensitivity of a similar device employing Agof irregular texture.

It is also possible to combine the concept of the rear reflective layerconsisting of a metal layer and a transparent layer, and the concept oftexture structure. U.S. Pat. No. 4,419,533 discloses the concept of therear reflective layer in which the surface of the metal layer has atextured structure and a transparent layer is formed thereon. The lightconversion efficiency of the solar cell, constituting an example of thephotovoltaic device, is expected to be significantly improved by suchcombination. However, according to the experiences of the presentinventors, such improvements have not been attained in most cases. Also,under certain conditions of semiconductor deposition, the obtained solarcell does not have enough reliability for use under the conditions ofhigh temperature and high humidity, despite the presence of thetransparent layer. For these reasons, the thin film solar cells have notbeen employed in practical applications, though they have thepossibility of low cost production.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a photovoltaic devicecomprising a metal with a smooth surface, a transparent layer formed onsaid smooth surface, and a photoelectric conversion layer formed on saidtransparent layer, wherein a surface of said transparent layer, oppositeto said smooth surface, has an irregular texture.

Another object of the present invention is to provide a photovoltaicdevice in which said transparent layer includes plural layers.

Still another object of the present invention is to provide aphotovoltaic device wherein said transparent layer contains an impurityfor increasing the electrical resistance.

Still another object of the present invention is to provide aphotovoltaic device wherein said transparent layer is composed of ZnO,and said impurity is composed of Cu.

Still another object of the present invention is to provide aphotovoltaic device wherein said transparent layer is composed of SnO₂,and said impurity is composed of Al.

Still another object of the present invention is to provide aphotovoltaic device wherein, in said plural layers of the transparentlayer, a layer at the side of the metal has a higher specificresistivity than in a layer at the side of the photoelectric conversionlayer.

Still another object of the present invention is to provide aphotovoltaic device in which said layer at the side of the metal isthicker than said layer at the side of the photoelectric conversionlayer.

Still another object of the present invention is to provide aphotovoltaic device in which a first layer of the transparent layer, incontact with said smooth surface, has an irregular texture on a surfacethereof opposite to said smooth surface.

Still another object of the present invention is to provide aphotovoltaic device in which said smooth surface has an irregulartexture with a pitch not exceeding 1000 Å, and the irregularly texturedsurface of said first layer has an irregular texture with a pitch withina range from 3000 to 20,000 Å and irregularities of 500 Å.

Still another object of the present invention is to provide aphotovoltaic device in which said transparent layer includes plurallayers, wherein at least a layer provided in contact with the smoothsurface of said metal has a smooth surface at the side of saidphotoelectric conversion layer, and at least a layer other than theabove-mentioned layer has an irregular texture on a surface at the sideof said photoelectric conversion layer.

Still another object of the present invention is to provide aphotovoltaic device in which said smooth surface has an irregulartexture with a pitch not exceeding 1000 Å, said at least a layer incontact with the metal has an irregular texture with a pitch of 3000 Åor less on the smooth surface thereof at the side of said photoelectricconversion layer, and said layer with irregular texture has an irregulartexture with a pitch within a range of 3000 to 20,000 Å andirregularities within a range of 500 to 20,000 Å.

Still another object of the present invention is to provide aphotovoltaic device in which a surface of said photoelectric conversionlayer, opposite to said transparent layer, has an irregular texturecomparable to that of said transparent layer.

Still another object of the present invention is to provide a method forproducing a photovoltaic device including a metal with a smooth surface,a transparent layer and a photovoltaic layer, which comprises utilizingthe difference in layer forming temperatures in the case where saidtransparent layer includes plural layers and a layer in contact with thesmooth surface of said metal has an irregular surface texture, and, forsaid layer forming temperatures represented as T1 for a first layer, T2for a second layer etc. from the side of said metal, satisfying areaction T1>Tn between said first layer forming temperature T1 and aforming temperature Tn for the second or subsequent layer.

Still another object of the present invention is to provide a method forproducing a photovoltaic device including a metal with a smooth surface,a transparent layer and a photovoltaic layer, which comprises utilizingthe difference in layer forming temperatures in the case where saidtransparent layer includes plural layers and at least a layer other thanthe layer in contact with said smooth surface of the metal has anirregular surface texture, and, for said layer forming temperaturesrepresented as T1 for the first layer, T2 for the second layer, etc.from the side of said metal, satisfying a relation T1<Tn between saidfirst forming temperature and a forming temperature Tn for the second orsubsequent layer.

Still another object of the present invention is to provide a method forproducing a photovoltaic device which comprises forming an irregularsurface texture in said transparent layer by immersing the surface ofsaid layer in acid, alkali, or aqueous salt solution, after thedeposition of a layer which is to be given said irregular surfacetexture.

Still another object of the present invention is to provide a method forproducing a photovoltaic device wherein said acid is acetic acid,sulfuric acid, hydrochloric acid, nitric acid, or perchloric acid, saidalkali is sodium hydroxide, potassium hydroxide, or aluminum hydroxide,and said salt is iron chloride or aluminum chloride.

Still another object of the present invention is to provide a secondarybattery wherein a photovoltaic device is provided on the batterycontainer and a reverse current-blocking diode is connected to at leastone electrode of said photoelectric conversion layer.

Still another object of the present invention is to provide anartificial satellite comprising a photovoltaic device formed on awindable substrate, a rotary shaft for winding said photovoltaic device,and a power source for driving said rotary shaft.

Still another object of the present invention is to provide a roofingmaterial comprising a corrugated plate, a plurality of photovoltaicdevices formed on said corrugated plate, a conductive sheet, and aresin, wherein electric connection between said photovoltaic devices ismade by said conductive sheet and a surface of said photovoltaicdevices, opposite to said corrugated plate, is covered by said resin.

Still another object of the present invention is to provide aphotovoltaic module comprising a plurality of photovoltaic devices andcurrent-collecting electrodes, wherein serial connections of saidphotovoltaic devices have mutually equal light-receiving areas and thedensity of said current-collecting electrodes is elevated in the casewhere many of said photovoltaic devices are serially connected.

Still another object of the present invention is to provide saidphotovoltaic device module wherein the thickness of the transparentelectrode constituting said photovoltaic devices is varied.

Still another object of the present invention is to provide anautomobile comprising a photovoltaic device module provided on a rearquarter pillar.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of an embodiment of thephotovoltaic device of the present invention, wherein the transparentlayer is composed of a first layer only;

FIG. 2 is a schematic cross-sectional view of an embodiment of thephotovoltaic device of the present invention, wherein a first layer hasan irregular texture;

FIG. 3 is a schematic cross-sectional view of an embodiment of thephotovoltaic device of the present invention, wherein a second layer hasan irregular texture;

FIGS. 4A and 4B are graphs showing the effect of ZnO on reflectance atthe interface between silicon and metal, in the absence (FIG. 4A) andpresence (FIG. 4B) of ZnO;

FIG. 5 is a chart showing the improvement in spectral sensitivity of aphotovoltaic device by an irregular surface texture;

FIG. 6 is a schematic cross-sectional view of a sputtering apparatus,adapted for producing the rear reflective layer of the presentinvention;

FIG. 7 is a view of a sputtering apparatus adapted for producing a rearreflective layer in the case where the transparent layer of the presentinvention consists solely of a first layer;

FIG. 8 is a schematic cross-sectional view of a sputtering apparatusadapted for producing a rear reflective layer, in the case where thefirst layer of the present invention has an irregular surface texture;

FIG. 9 is a schematic cross-sectional view of a sputtering apparatusadapted for producing a rear reflective layer, in the case where thesecond layer of the present invention has an irregular surface texture;

FIG. 10 is a schematic cross-sectional view of another embodiment of thephotovoltaic device of the present invention, wherein the transparentlayer is composed of a first layer only;

FIG. 11 is a schematic cross-sectional view of another embodiment of thephotovoltaic device of the present invention, wherein a first layer hasan irregular texture;

FIG. 12 is a schematic cross-sectional view of another embodiment of thephotovoltaic device of the present invention, wherein a second layer hasan irregular texture;

FIG. 13A and 13B are respectively a schematic view and a schematicexploded view of a photovoltaic device of the present invention, appliedto a secondary battery;

FIG. 14 is an equivalent circuit diagram of an application of thesecondary battery, utilizing the photovoltaic device of the presentinvention;

FIG. 15 is a schematic view of a connecting part of a solar cell for anartificial satellite, utilizing the photovoltaic device of the presentinvention;

FIG. 16 is a schematic view of an artificial satellite employing thephotovoltaic device of the present invention;

FIG. 17 is a schematic magnified view of a corrugated roofing materialemploying the photovoltaic device of the present invention;

FIG. 18 is a schematic view of a corrugated roofing material employingthe photovoltaic device of the present invention;

FIG. 19 is a schematic view of a solar cell module employing thephotovoltaic device of the present invention and having acurrent-collecting electrode density which is variable according to thearea of the device;

FIG. 20 is a schematic view of an automobile utilizing the photovoltaicdevice of the present invention; and

FIG. 21 is an equivalent circuit diagram of an application of thephotovoltaic device of the present invention in an automobile.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention has been attained in consideration of theforegoing, and provides a photovoltaic device of high conversionefficiency and high reliability through the use of an improved rearreflective layer and a method for producing the same, and also providesa secondary battery, an artificial satellite, a roofing material, asolar cell module, and an automobile or the like utilizing saidphotovoltaic device.

The present inventors have found that the conventional rear reflectivelayer has been associated with the following drawbacks:

(1) Loss of reflectance due to irregular texture of the metal layer:

On the metal surface with the irregular texture, the light is randomlyreflected in various directions. However, measurement with anintegrating sphere capable of collecting the reflected light in alldirections indicates that the metal layer with irregular surfacestructure shows a considerable loss in reflectance, in comparison withthe smooth metal layer. This tendency is particularly conspicuous withAl or Cu. For this reason it is not possible to effectively return thelight, transmitted through the semiconductor layer, back to saidsemiconductor layer, and the conversion efficiency of the photovoltaicdoes not reach, therefore, the expected level;

(2) Metal diffusion into the transparent layer:

The deposition of the semiconductor layer on the rear reflective layeris usually conducted with a substrate temperature of 200° C. or higher.At such temperature, the metal atoms of the rear reflective layerpenetrate the overlying transparent layer and diffuse to the surfacethereof. Such direct contact of the metal with the semiconductor layerresults in insufficient function of the transparent layer, leadingeventually to lowered reliability;

(3) Metal diffusion to the semiconductor layer:

In the formation of the transparent layer over the rear reflectivelayer, the underlying metal layer may be locally exposed. Suchphenomenon is encountered particularly when the irregular texture of themetal layer is enlarged and the transparent layer is made thinner. Ifthe semiconductor layer is deposited thereon, the metal atoms diffusefrom the exposed area of the metal layer to the semiconductor layer,thus affecting the properties of the semiconductor junction;

(4) Drawbacks in the subsequent steps:

The semiconductor layer contains defects such as pinholes, through whichdirect contact can be formed between the upper electrode on thesemiconductor layer and the underlying transparent layer. Excessivecurrent will result in such directly contacting portions, unless thetransparent layer has a suitable resistance.

Also, if the metal layer has an exposed portion as mentioned in (3),there may result direct contact between the upper electrode and the rearelectrode through defects, such as pinholes, of the semiconductor layer.

Through the investigation of these drawbacks, the present inventors havereached the basic concept of the present invention, to be explained inthe following. There is provided a photovoltaic device comprising ametal layer having a substantially smooth surface for reflecting light,a transparent layer provided on said surface and having an irregularsurface at a side opposite to said smooth surface, a photoelectricconversion layer consisting of a semiconductor and provided on saidtransparent layer, and a transparent electrode layer provided on saidphotoelectric conversion layer, wherein said transparent layer iscomposed of plural layers, and a layer provided in contact with saidmetal layer having an irregular surface at the side of saidphotoelectric conversion layer.

Also, there is provided a photovoltaic device wherein said transparentlayer is composed of plural layers, in which a layer provided in contactwith said metal layer has a substantially smooth surface at the side ofsaid photoelectric conversion layer and at least another of said layershas an irregular surface at the side of said photoelectric conversionlayer.

FIG. 1 shows an example of the configuration of the photovoltaic deviceof the present invention. On conductive substrate 101, there is formedmetal layer 102 of high reflectance. The metal layer 102 may bedispensed with if the substrate itself is composed of a material ofsufficiently high reflectance.

The surface of the metal layer 102 is a smooth surface with a flatnessnot affecting the specific reflectance of the metal itself, namely themagnitude of any irregularities does not exceed 1000 Å. Transparentlayer 103 is formed thereon. It is transparent to the light transmittedby the photoelectric conversion layer, has a suitable electricresistance, and has an irregular surface structure. In the case wherethe transparent layer contains plural layers (FIG. 2 showing an examplein which the transparent layer has a two-layered structure composed offirst layer 103a and second layer 103b), the surface of the first layer103a has an irregular surface structure with the magnitude ofirregularities being in the range of 500 to 20,000 Å and the pitch ofirregularities being in the range of 3000 to 20,000 Å. The second layer103b formed thereon may have a considerably higher specific resistivitythan the first layer 103a. In such case, however, the second layer 103bhas to be as thin as possible, in order to reduce the resistance perunit area of the photovoltaic device. In the case where the transparentlayer is composed of a plural layers (FIG. 3 showing an example with atwo-layered structure composed of first layer 103c and second layer103d) wherein the first layer 103c has a smooth light-receiving surfacewith the pitch of the irregularities not exceeding 3000 Å and has asuitable electrical resistance while the second layer 103d is subjected,for the formation of surface irregularities, to the action of an aqueoussolution of acid, alkali, or salt as will be explained later, there isemployed first layer 103c less eroded by said aqueous solution thansecond layer 103d. The second layer 103d has an irregular surfacestructure with a pitch of irregularities of 3000 to 20,000 Å and aheight of irregularities of 500 to 20,000 Å, and has light transmissioncharacteristics and electrical properties similar to those of the firstlayer.

Said transparent layer also has chemical resistance to the etchant orthe like to be employed in the subsequent steps. Semiconductor junction104 is provided thereon. As an example of the semiconductor junction,there is illustrated a PIN a-Si photovoltaic device, consisting ofn-type a-Si layer 105, i-type a-Si layer 106, and p-type a-Si layer 107.If the semiconductor junction layer 104 is thin, the entire layer oftenassumes an irregular structure similar to that of the transparent layer103, as illustrated in FIGS. 1, 2, and 3. Transparent electrode 108 isformed thereon, and comb-shaped current-collecting electrode 109 isprovided thereon. The above-mentioned configuration provides thefollowing advantages:

(1) Because the surface of the metal layer 102 (or the substrate 101itself) is smooth, the light reflectance on the metal surface iselevated. Besides, the irregular surface structure of the transparentlayer 103 (and of the semiconductor junction layer 104) induces lightscattering, due to the phase difference at the peaks and valleys of theirregularities, at the interface with said transparent layer, therebycausing a light trapping effect inside the semiconductor junction layer104. For this reason the incident light is effectively absorbed, and theconversion efficiency of the photovoltaic device is improved;

(2) The smooth surface of the metal layer 102 (or the substrate 101itself) reduces the contact area with the transparent layer 103, therebyreducing the diffusion of the metal atoms into the transparent layer103.

Also, since the transparent layer 103 has an appropriate electricalresistance, excessive current is not generated even if the photoelectricconversion layer 104 develops defects. Further, as the transparent layer103 is chemically resistant, the rear reflective layer is damaged lessin the subsequent process;

(3) Even if the metal layer 102 is partially exposed in the formation ofthe irregular surface structure on the first layer 103a, the probabilityof contact between the upper electrode and the metal layer 102 isdrastically reduced because of the covering by the second layer 103b,whereby the reliability of the photovoltaic device is significantlyimproved;

(4) Also, since the smooth first layer 103c is deposited on the smoothmetal layer 102, the frequency of partial exposure of the metal layer102, that can be encountered when the metal layer is given the irregularsurface structure, is extremely reduced, whereby the diffusion of themetal atoms from the metal layer 102 to the semiconductor layer 104 canbe prevented;

(5) Also, since the smooth first layer 103c is deposited on the smoothmetal layer 102, the frequency of partial exposure of the metal layer102, that can be encountered when the metal layer has an irregularsurface structure, is extremely reduced, whereby a photoelectric deviceof high reliability is obtained, with little contact between the upperelectrode and the metal layer.

In the following there will be explained experiments for verifying theeffects of the present invention.

Experiment 1

On a 5×5 cm stainless steel plate (SUS 430), Al was deposited to athickness of 1500 Å by DC magnetron sputtering, with the substrate atroom temperature. Then ZnO was deposited to a thickness of 4000 Å by DCmagnetron sputtering, with a substrate temperature of 250° C. Under SEMobservation, the Al surface was smooth and lustrous, while the ZnOsurface was turbid, showing concentrated crater-shaped recesses withdiameters in a range of 4000 to 6000 Å. In this state the lightreflectance was measured within a wavelength range of 6000 to 9000 Å. Onthe thus-formed rear reflective layer, there was formed a semiconductorjunction layer, by means of glow discharge decomposition, an n-type a-Silayer of a thickness of 200 Å utilizing SiH₄ and PH₃ as the raw materialgases, an i-type a-Si layer of a thickness of 4000 Å utilizing SiH₄ asthe raw material gas, and a p-type microcrystalline (μc) Si layer of athickness of 100 Å utilizing SiH₄, BF₃, and H₂ as the raw materialgases. (Amorphous silicon obtained by glow discharge decomposition ofSiH₄, etc. is generally written as a-Si:H because it contains hydrogenin about 10%, but, in the present text, it is represented as a-Si forthe purpose of simplicity.) An ITO film of a thickness of 650 Å wasdeposited thereon by resistance-heated vapor deposition, and acurrent-collecting electrode of a width of 300 microns was formedthereon by silver paste, thereby obtaining Sample 1a.

Sample 1b was prepared in the identical manner as Sample 1a, except thatthe Al deposition was conducted with a substrate temperature of 300° C.

Also, Sample 1c was prepared in the identical manner as Sample 1a,except that the ZnO was conducted with the substrate at the roomtemperature.

Also, Sample 1d was prepared in the identical manner as Sample 1a,except that the stainless steel substrate was employed withoutdeposition of Al or ZnO.

Also, Sample 1e was prepared in the identical manner as Sample 1a,except that the stainless steel substrate was replaced by an A1substrate of the same size, the surface of which was polished to a pitchof irregularities of about 1000 Å and Al deposition was not conducted.

The five samples thus obtained were subjected to the measurement ofconversion efficiency as photoelectric conversion devices, in a solarsimulator of AM-1.5 light. The obtained results are shown in Table 1,from which the following conclusions can be deduced:

(1) The conversion efficiency is improved with any rear reflectivelayer, in comparison with the absence of the rear reflective layer;

(2) The highest efficiency with a rear reflective layer is obtained whenthe Al layer had a smooth surface and the ZnO layer had an irregularsurface texture; and

(3) The polished Al substrate has an equivalent effect as the smooth Allayer.

Experiment 2

Sample 2a was obtained in the same manner as Sample 1a in Experiment 1,except that A1 was replaced by Ag and the current-collecting electrodewas not formed.

Also, Sample 2b was prepared in the same manner as Sample 2a, exceptthat the Ag deposition was conducted with a substrate temperature of350° C. instead of room temperature.

In Sample 2a, the entire rear reflective layer lacked lustre because theZnO layer had an irregular surface structure although the Ag surface wassmooth. In Sample 2b, the Ag surface had an irregular structure.

Table 2 shows the results of measurement of the conversion efficiency ofboth samples in a solar simulator of AM-1.5 light. Sample 2b showed avery low conversion efficiency, which is attributed to ashort-circuiting, in consideration of the current-voltagecharacteristics. Also, in the SEM observation of both samples, Sample 2bshowed scattered spot-shaped defects, and Auger analysis of these defectlocations indicated Ag diffusion to the surface.

Experiment 3

Sample 3a was prepared in the same manner as Sample 1a in Experiment 1,except that a-Si deposition was conducted in a deposition chamber whichhad been used for film deposition for a prolonged period without chambercleaning.

Also, Sample 3b was prepared in the same manner as Sample 3a, exceptthat the ZnO deposition was conducted with a ZnO target containing 0.5%of Cu.

Table 3 shows the results of measurement of the conversion efficiency ofboth samples in a solar simulator of AM-1.5 light. Both samples showedlower conversion efficiencies than in Sample 1a, under the influence ofshort-circuiting, particularly conspicuous in Sample 3a. In the SEMobservation, both samples showed many pinholes in the a-Si layer. Forreference, the resistivity of ZnO, measured by forming a chromium (Cr)electrode directly on the surface of the rear reflective layer andproviding a weak current, was 5×10² Ωcm in Sample 3a and 2×10⁵ Ωcm inSample 3b. It is therefore considered that, in Sample 3b, the specificresistivity of ZnO was suitably elevated, thereby suppressing thecurrent in the pinholes.

Experiment 4

Sample 4a was prepared in the same manner as the rear reflective layerof Sample 1a in Experiment 1, except that the ZnO deposition wasconducted with a ZnO target containing 0.5% of Cu. Also the rearreflective layer of Sample 1a was taken as Sample 4b. Both samples wereimmersed, for 5 minutes, in a 30% aqueous solution of ferric chloride,which is the etchant employed for ITO patterning. Sample 4a did not showany particular change, but, in Sample 4b, ZnO was significantlycorroded. From these results it is expected that the rear reflectivelayer of Sample 4a is less prone to be damaged in the subsequent steps,even if the thin semiconductor films have defects such as pinholes.

Experiment 5

Sample 5 was prepared in the same manner as Sample 1a in Experiment 1,except that the deposition of the i-type a-Si layer was conducted with atripled power of glow discharge and with a 1/3 flow rate of SiH₄,combined with deposition conditions so regulated as to obtain a filmthickness of 4000 Å. The SEM observation of Samples 1a, 5 and of therear reflective layers employed thereon provided the following results.The surface of Sample 1a showed concentrated crater-shaped recesses withdiameters of 4000 to 6000 Å on the surface of the rear reflective layer,with almost the same depth of craters on both surfaces. On the otherhand, the surface of Sample 5 showed heaved structures of a pitch finerthan the recesses on the rear reflective layer, apparently indicating anaspect different from the structure of the rear reflective layer.

In an evaluation in the solar simulator, Sample 5 showed a conversionefficiency of 8.2%, lower than that of Sample 1a. This difference isprincipally due to a low short-circuit photocurrent, indicating aninsufficient phototrapping effect in the case where, as in Sample 5, theupper surface structure of the semiconductor layer is different fromthat of the rear reflective layer.

Experiment 6

On a 5×5 cm stainless steel plate (SUS 430), Al was deposited to athickness of 1500 Å by DC magnetron sputtering, with the substrate atroom temperature. Then ZnO was deposited thereon to a thickness of 4000Å by magnetron sputtering, with a substrate temperature of 300° C. UnderSEM observation, the Al surface was lustrous and smooth with a pitch ofirregularities of 1000 Å or less, while the ZnO surface was turbid, withconcentrated crater-shaped recesses of diameters of 4000 to 9000 Å.

The height of the irregularities was about 2000 to 4000 Å. ZnO wasfurther deposited thereon to a thickness of 500 Å, by magnetronsputtering with the substrate at room temperature. Under SEMobservation, this layer exhibited an irregular surface structure similarto that of the first layer formed with the substrate temperature of 300°C. In this state the light reflectance was measured in a wavelengthrange of 6000 to 9000 Å. Subsequently, there were formed thereon asemiconductor junction, a transparent electrode, and acurrent-collecting electrode to obtain Sample 6a.

Also, Sample 6b was prepared in the same manner as Sample 6a, exceptthat the A1 deposition was conducted with a substrate temperature of100° C.

Also, Sample 6c was prepared in the same manner as Sample 6a, exceptthat the ZnO of the second layer (at the side of the semiconductorlayer) was not deposited.

Also, Sample 6d was prepared in the same manner as Sample 6a, exceptthat ZnO of the first layer was deposited to a thickness of 4500 Å witha substrate temperature of 300° C., and ZnO of the second layer was notdeposited.

Also, Sample 6e was prepared in the same manner as Sample 6a, exceptthat ZnO of the first layer was deposited to a thickness of 4000 Å withthe substrate at room temperature, and ZnO of the second layer was notdeposited.

Also, Sample 6f was prepared in the same manner as Sample 6a, exceptthat ZnO of the first layer was deposited to a thickness of 4500 Å withthe substrate at room temperature, and ZnO of the second layer was notdeposited.

Also, Sample 6g was prepared in the same manner as Sample 6a, exceptthat the stainless steel substrate was replaced by an A1 substrate ofthe same size, the surface of which was polished to a pitch ofirregularities of about 1000 Å.

The seven samples thus prepared were subjected to the measurement ofconversion efficiency as solar cells, as an example of the photovoltaicdevice, in a solar simulator of AM-1.5 light. The obtained results areshown in Table 4, from which the following conclusions are deduced:

(1) The conversion efficiency was improved when a smooth metal layer(pitch of irregularities not exceeding 1000 Å) was combined with atransparent layer with an irregular surface structure (Samples 6a, 6c,6d, and 6g); and

(2) Samples 6a, 6c, 6d, and 6g were substantially the same in conversionefficiency, but the one-layered transparent layer (6c, 6d) and thetwo-layered transparent layer (6a, 6g) respectively provided yields ofphotovoltaic devices of about 70% and about 95%, with apparentlyimproved reliability in the two-layered structure (said yield beingcalculated by the shunt resistance per unit area (1 cm²), determinedfrom the measurement of the current-voltage characteristics of eachsample, each sample being rated as acceptable if said shunt resistanceis equal to 200 Ωcm or higher and not acceptable below 200 Ωcm).

This difference is presumably due to the following phenomena. In theone-layered structure, in the formation of irregular structures in thetransparent layer, the recesses locally grew larger, thus partiallyexposing the metal layer and causing electrical short-circuiting throughsuch exposed areas when the photovoltaic device is formed. On the otherhand, in the two-layered structure, said exposed areas are effectivelycovered by the second layer, whereby the frequency of theshort-circuiting is reduced and the yield is increased.

Experiment 7

Sample 7a was prepared in the same manner as Sample 6a in Experiment 6,except that ZnO of the first layer was deposited to a thickness of10,000 Å and the substrate was thereafter immersed in 10% aqueoussolution of acetic acid for 1 minute at room temperature.

Also, Sample 7b was prepared in the same manner as Sample 7a except thatZnO of the first layer had a thickness of 25,000 Å.

Also, Sample 7c was prepared in the same manner as Sample 7b, exceptthat the immersion of the substrate in 10% aqueous acetic acid solutionwas conducted for 90 seconds.

Also, Sample 7d was prepared in the same manner as Sample 7b, exceptthat the immersion of the substrate in 10% aqueous acetic acid solutionwas conducted for 3 minutes.

SEM observation showed that the magnitude of the irregular structure ofthe first layer of ZnO transparent layer grew with the immersion time,due to the etching action of the acetic acid solution.

Under SEM observation, Samples 7a and 7d showed locally exposed areas ofthe metal layer immediately after immersion in the acetic acid solution,but such local exposures of the metal layer could no longer be observedafter the deposition of the second layer of the ZnO transparent layer.Table 5 shows the results of measurement of conversion efficiency of thefour samples under AM-1.5 light. Samples 7a, 7b and 7c showed highconversion efficiencies but Sample 7d did not show such high conversionefficiency.

Experiment 8

Sample 8a was prepared in the same manner as Sample 6a, except that thedeposition of the transparent layer was not conducted.

Under AM-1.5 light, Sample 8a showed a conversion efficiency of 2.2%.Auger microanalysis of the a-Si layer of Sample 8a detected aluminumatoms.

It was therefore confirmed that the direct contact between the metallayer and the semiconductor layer induced diffusion of metal atoms intothe semiconductor layer.

Experiment 9

On a 5×5 cm stainless steel substrate (SUS 430), Al was deposited to athickness of 1500 Å by DC magnetron sputtering, with the substrate atroom temperature. Then ZnO was deposited to a thickness of 1000 Å by DCmagnetron sputtering, also with the substrate at room temperature. UnderSEM observation, the Al surface was a lustrous smooth surface with apitch of irregularities of 1000 Å or less. Also, the ZnO layer had alustrous smooth surface, though yellowish, with a pitch ofirregularities of 1000 Å or less. Then, ZnO was deposited thereon to athickness of 3000 Å by DC magnetron sputtering, with a substratetemperature of 300° C. Under SEM observation, the ZnO film constitutingthe second layer had a turbid surface, with a pitch of irregularities of4000 to 8000 Å and a magnitude of irregularities of 2000 to 3000 Å. Inthis state, the light reflectance was measured in a wavelength range of6000 to 9000 Å.

Subsequently, there were formed thereon a semiconductor junction, atransparent electrode, and a current-collecting electrode in the samemanner as in Experiment 1, whereby Sample 9a was obtained.

Also, Sample 9b was prepared in the same manner as Sample 9a, exceptthat the A1 deposition was conducted with a substrate temperature of100° C.

Also, Sample 9c was prepared in the same manner as Sample 9a, exceptthat ZnO of the second layer was not deposited.

Also, Sample 9d was prepared in the same manner as Sample 9a, exceptthat ZnO of the first layer was deposited to a thickness of 4000 Å withthe substrate at room temperature and ZnO of the second layer was notdeposited.

Also, Sample 9e was prepared in the same manner as Sample 9a, exceptthat ZnO of the first layer was deposited to a thickness of 4000 Å witha substrate temperature of 300° C. and the second ZnO layer was notdeposited.

Also, Sample 9f was prepared in the same manner as Sample 9a, exceptthat the stainless steel substrate was replaced by an Al substrate ofthe same size, the surface of which was polished to a pitch ofirregularities of 1000 Å or less, and Al deposition was not conducted.

The six samples thus obtained were subjected to the measurement ofconversion efficiency as solar cells, as an example of the photovoltaicdevice, in a solar simulator of AM-1.5 light. The obtained results areshown in Table 6, from which the following conclusions were deduced:

(1) The conversion efficiency was improved by the use of a rearreflective layer, composed of a combination of a smooth metal layer anda transparent layer with irregular surface structure (Samples 9a, 9e,and 9f);

(2) Although Samples 9a, 9e, and 9f were almost the same in conversionefficiency, the one-layered transparent layer (Sample 9e) provided ayield of the photovoltaic devices of about 70% while Samples 9a and 9fwith the two-layered transparent layer provided yields of about 95%.Thus the two-layered structure was apparently higher in yield andreliability. (Said yield was calculated by the shunt resistance per unitarea (1 cm²), determined from the measurement of the current-voltagecharacteristics of each sample, and each sample was rated as acceptableif said shunt resistance is equal to 200 Ωcm and not acceptable below200 Ωcm.) This difference is presumably due to the following phenomena.In the one-layered structure, in the formation of irregular structure inthe transparent layer, the recesses locally grew larger, thus partiallyexposing the metal layer and causing electrical short-circuiting betweenthus-exposed areas and the upper electrode through defects such aspinholes in the semiconductor layer formed thereon. On the other hand,the use of a two-layered transparent layer in which the first layer hasa smooth surface drastically reduces the formation of said exposedareas, thereby reducing the formation of electrical short-circuits andimproving the yield.

Experiment 10

Sample 10a was prepared in the same manner as Sample 9a, except that Alin Experiment 9 was replaced by Ag and the current-collecting electrodewas not formed.

Also, Sample 10b was prepared in the same manner as Sample 10a, exceptthat the Ag deposition was conducted with a substrate temperature of300° C. instead of room temperature.

In Sample 10a, the rear reflective layer lacked lustre, because the ZnOhad an irregular surface structure through the Ag had a smooth surface.In Sample 10b, the Ag surface showed an irregular structure.

Table 7 shows the result of measurement of conversion efficiency underAM-1.5 light of both samples. Sample 10b showed a significantly lowconversion efficiency, presumably due to short-circuiting, based on thecurrent-voltage characteristics. Under SEM observation of both samples,Sample 10b showed scattered spot-shaped defects, and Auger analysis ofthese defects revealed Ag diffusion to the surface.

Experiment 11

In the process of Experiment 9, SnO₂ was deposited as the first layer toa thickness of 1000 Å by DC magnetron sputtering, with a substratetemperature of 200° C. The SnO₂ showed a lustrous smooth surface, with apitch of irregularities not exceeding 1000 Å. Then ZnO was depositedthereon, as the second layer, with a thickness of 10,000 Å, with asubstrate temperature of 300° C. Sample 11a was obtained by a subsequentprocess same as that for Sample 9a, except that the substrate surfacewas immersed in a 20% aqueous solution of perchloric acid for 30 secondsat room temperature.

Also, Sample 11b was obtained in the same manner as Sample 11a, exceptthat the immersion of the substrate surface in the 20% aqueous solutionof perchloric acid was conducted for 45 seconds.

Also, Sample 11c was prepared in the same manner as Sample 11a, exceptthat ZnO of the second layer was deposited to a thickness of 25,000 Å.

Also, Sample 11d was prepared in the same manner as Sample 11c, exceptthat the immersion of the substrate surface in the 20% aqueous solutionof perchloric acid was conducted for 45 seconds.

Also, Sample 11e was prepared in the same manner as Sample 11c, exceptthat the immersion of the substrate surface in the 20% aqueous solutionof perchloric acid was conducted for 90 seconds.

SEM observation showed that the irregularities of the irregular surfacestructure of ZnO constituting the second layer grew larger with theimmersion time, by the etching action of the aqueous perchloric acidsolution.

In comparison with the irregularities of Sample 11d, those of Sample 11bare smaller. This is presumably due to the weaker action of the aqueousperchloric acid solution to SnO₂ than to ZnO.

Table 8 shows the result of measurement of conversion efficiency ofthese five samples under AM-1.5 light. Although Samples 11a to 11dshowed high conversion efficiencies, Sample 11e did not show a highconversion efficiency.

In the following there will be given a detailed explanation of the rearreflective layer employed in the photovoltaic device of the presentinvention.

Substrate and metal layer

Various metals can be employed as the substrate. In particular,stainless steel plate, zincated steel plate, aluminum plate, and copperplate are preferred because of the relatively low cost. The metal platemay be cut into a predetermined shape, or may be employed as acontinuous web, depending on the thickness. The latter form, beingwindable into a coil, is suitable for continuous production, andfacilitates storage and transportation. Also, for certain applicationsthere may be employed a crystalline substrate such as of silicon, or aplate of glass or ceramics. The substrate surface may be polished, or itmay be used without polishing if the surface finish is satisfactory, asin bright annealed stainless steel plate.

A substrate with a low light reflectance such as stainless steel plateor zincated steel plate, or a substrate with a low conductivity such asglass or ceramic plate can be used as the substrate of the presentinvention, by forming thereon a metal layer of high reflectance such asof silver, aluminum, or copper. However, since the shorter wavelengthcomponent in the solar light spectrum is already absorbed by thesemiconductor layer, the rear reflective layer needs only to have a highreflectance of the light of the longer wavelength region. The shortestboundary wavelength of such high reflectance region depends on theoptical absorption coefficient and the thickness of the semiconductormaterial to be employed. As an example, in the case of a-Si with athickness of 4000 Å, said boundary wavelength is about 6000 Å, so thatcopper can be advantageously used.

The metal layer deposition, if employed, can be achieved, for example,by vacuum evaporation by means of resistance heating or with an electronbeam, sputtering, ion plating, CVD, or plating. Sputtering will beexplained in the following, as an example of a film forming method. FIG.6 shows an example of the sputtering apparatus, in which depositionchamber 401 can be evacuated by a vacuum pump (not shown). Inert gas,such as argon (Ar), is introduced into said chamber, with apredetermined flow rate from gas supply pipe 402 connected to a gascylinder (not shown), and the interior of the deposition chamber 401 ismaintained at a predetermined pressure by the adjustment of the apertureof exhaust valve 403. Substrate 404 is fixed on anode 406, containingheater 405 therein. Opposed to the anode 406 there is provided cathode408, supporting thereon target 407, which is a block of the metal to bedeposited. It is usually composed of the metal of a purity of 99.9 to99.999%, but a specific impurity may be contained in certain cases. Thecathode is connected to power source 409, which applies an RF (radiofrequency) or DC high voltage, thereby generating plasma 410 between thecathode and the anode. The metal atoms of the target 407 are depositedonto the substrate 404, by the action of said plasma. A higherdeposition rate can be obtained in a magnetron sputtering apparatus inwhich a magnet is provided in the cathode 408 for increasing the plasmaintensity.

An example of the depositing conditions is as follows. There wereemployed an Al target of a purity of 99.99% and a diameter of 6 inches;a substrate consisting of stainless steel plate (SUS 430) of a size of5×5 cm and a thickness of 1 mm, with a polished surface; a distance of 5cm between the target and the substrate; and a flow rate of Ar of 10sccm, with a pressure of 1.5 mTorr. A DC voltage of 500 V generatedplasma with a current of 2 A, and the discharge was maintained in thisstate for 1 minute. Samples 13a, 13b, 13c, and 13d were obtainedrespectively at substrate temperatures of room temperature, 100° C.,200° C., and 300° C. Table 9 summarizes the appearance and the result ofSEM observation of these samples. The Al surface apparently varied fromsmooth surface to irregular surface, with the increase in temperature. Asimilar tendency is generally observed with other metals and in otherfilm forming methods.

Transparent Layer and its Irregular Structure

The transparent layer is often composed of oxides such as ZnO, In₂ O₃,SnO₂, CdO, CdSnO₄, or TiO, though the actual composition of thesecompounds does not necessarily coincide with that of the chemicalformula. The optical transmittance of the transparent layer shouldpreferably be as high as possible, but it need not be transparent to thelight of the wavelength region which is absorbed by the semiconductor,or which is not absorbed by the semiconductor. The transparent layershould preferably have a certain electrical resistance in order tosuppress the short-circuit current generated, for example, by thepinholes, but said resistance should be such that it causes negligibleinfluence on the series resistance loss of the conversion efficiency ofthe photovoltaic device. Based on these considerations, the resistanceper unit area (1 cm²) of the transparent layer should preferably withina range from 10⁶ to 10 Ω, more preferably from 10⁻⁵ to 3 Ω, and mostpreferably from 10⁴ to 1 Ω. Also, the thickness of the transparent layeris preferably as small as possible in consideration of the transparency,but should be at least 500 Å in consideration of multiple interferenceeffects. Also, the average thickness should be at least 1000 Å forforming the irregular surface structure. A larger film thickness may berequired in consideration of the reliability. In the case of plurallayers, an average film thickness of at least 1000 Å is required inorder to form the irregular surface structure in the first layer.

The irregular structure can be formed in the first layer by elevatingthe temperature during the deposition of the layer in contact with themetal layer. In this case, the temperature T1 for forming the firstlayer varies suitably according to the material and apparatus employedin said formation, but is preferably higher than 200° C. in the DCmagnetron sputtering employing a ZnO target (purity 99.9%). Also, sincethe irregular surface structure of oxide or the like employed in saidtransparent layer grows as the forming temperature becomes higher, thereis employed a relation T1>Tn wherein Tn is the forming temperature ofthe n-th layer.

An alternative method for forming the irregular structure consists,after formation of the first layer, of immersing the surface thereof inaqueous solution of acid, alkali, or salt. A desired irregular structurecan be obtained by regulating the length of the immersion time. Thefrequently used examples of acid include acetic acid, sulfuric acid,hydrochloric acid, nitric acid, and perchloric acid, while those ofalkali include sodium hydroxide, potassium hydroxide, and aluminumhydroxide, and those of salt include ferric chloride and aluminumchloride.

Another alternative method for forming the irregular structure consists,after formation of the first layer, of bombarding the surface of thetransparent layer on which the irregular structure is to be formed, withplasma or ions, for example, by reverse sputtering. This method can beconducted in a relatively simple manner, and is suitable for batchprocessing. The second or any subsequent layer of the transparent layershould be of such thickness that it does not deteriorate the irregularstructure formed by the first layer. Further, in consideration of thethickness of the semiconductor layer to be deposited thereon, saidthickness is preferably within a range from 500 to 3000 Å, morepreferably from 500 to 2500 Å, and most preferably from 500 to 2000 Å.

The forming temperature Tn of at least the n-th layer of the second andsubsequent layers should be as low as possible, in order to form asmooth surface for completely covering the irregularities of the firstlayer. Said n-th forming temperature Tn varies suitably according to thematerial and apparatus employed in the formation of the n-th layer, buta condition Tn<200° C. is preferred, for example, in the DC magnetronsputtering employing a ZnO target (purity 99.9%).

Also, in another example composed of plural layers, the first formingtemperature T1 is preferably as low as possible, as a method for forminga pitch of irregularities not exceeding 3000 Å in the first layer.

Said first forming temperature T1 varies according to the material andapparatus employed in the formation of the first layer, but a conditionT1<200° C. is preferred, for example, in the DC magnetron sputteringemploying a ZnO target (purity 99.9%).

Among the second and subsequent layers in the transparent layer, then-th layer having the irregular surface structure should have an averagethickness of 1000 Å or larger, in order to have such irregularstructure. A method for forming said irregular structure consists ofelevating the n-th forming temperature Tn. Said temperature Tn variesdepending on the material and apparatus employed for the formation ofthe n-th layer, but a condition Tn>200° C. is preferred, for example, inthe DC magnetron sputtering employing a ZnO target (purity 99.9%). Also,since the irregular surface structure of the oxide or the like employedin said transparent layer grows larger as the forming temperature Tbecomes higher, there is employed a relation T1<Tn.

An alternative method for forming the irregular structure consists,after the deposition of the n-th layer in which said irregular structureis to be formed, of immersing the surface thereof in an aqueous solutionof acid, alkali, or salt. A desired irregular structure can be obtainedby regulating the length of the immersion time. Examples of such acidinclude acetic acid, sulfuric acid, hydrochloric acid, nitric acid, andperchloric acid, while those of said alkali include sodium hydroxide,potassium hydroxide, and aluminum hydroxide, and those of said saltinclude ferric chloride and aluminum chloride.

Another alternative method for forming the irregular structure consists,after the deposition of a transparent layer in which the irregularstructure is to be formed, of bombarding the surface thereof with plasmaor ions, for example, to effect sputtering thereof. This method can berelatively easily conducted, and is suitable for batch processing.

Among the second and subsequent layers in the transparent layer, then-th layer having said irregular structure and the subsequent layersdeposited thereon should have a total thickness which does notdeteriorate said irregular structure. Also, in consideration of thethickness of the semiconductor to be provided thereon, said totalthickness is preferably within a range from 500 to 3000 Å, morepreferably from 500 to 2500 Å, and most preferably from 500 to 2000 Å.

The transparent layer may be deposited by vacuum evaporation employingresistance heating or an electron beam, sputtering, ion plating, CVD, orspray coating. The sputtering method will be explained as an example ofa film forming method. Also in this case, the sputtering apparatus shownin FIG. 6 may be employed. In the case of oxide deposition, however,there may be employed a target of the oxide itself or of a metal (Zn,Sn, etc.). In the latter case, oxygen must be supplied, together withargon, to the deposition chamber ("reactive" sputtering).

In the following there are shown examples of the conditions fordeposition and irregular structure formation. There was employed astainless steel plate (SUS 430) of a size of 5×5 cm and a thickness of 1mm, with a polished surface, as the substrate. There was also employed aZnO target of a diameter of 6 inches, with a purity of 99.9%, atdistance of 5 cm between the target and the substrate. Argon wassupplied at a flow rate of 10 sccm and maintained at a pressure of 1.5mTorr, and a DC voltage of 500 V was applied to generate plasma, with acurrent of 1 A. The discharge was maintained in this state for 5minutes. Samples 14a, 14b, 14c, and 14d were obtained by respectivelymaintaining the substrate at room temperature, 100° C., 200° C., and300° C. Table 10 summarizes the appearances and the results of SEMobservation of these samples. The surface state of ZnO varied as thetemperature was elevated. The turbid samples 14c and 14d showedcrater-shaped recesses on the surface, which are assumed to be the causeof turbidity.

In the following there are shown examples of the conditions fordeposition and irregular structure formulation, in the case of plurallayers in which the first layer has an irregular surface structure.

There were employed the same conditions for 15 minutes. Samples 15a,15b, 15c, and 15d were prepared by maintaining the substrate at the roomtemperature, 100° C., 200° C., and 300° C., respectively. The surfacestate of the ZnO varied as the temperature was elevated. The turbidSamples 15c and 15d showed crater-shaped recesses on the surface, whichwere considered as the cause of the turbidity. Additional Samples 15eand 15f were prepared by immersing the sample, prepared at a substratetemperature of 200° C., in a 10% aqueous solution of acetic acid for 1and 1.5 minutes, respectively. Table 11 summarizes the appearances andthe result of SEM observation of these samples.

Samples 15a to 15f thus prepared were again subjected to the depositionof ZnO by sputtering under the same conditions as above but with adischarge time of 1.5 minutes, whereby there was obtained a depositionfilm of an irregular surface structure substantially the same as that ofSamples 15a to 15f.

In the following there are shown examples of the conditions fordeposition and irregular structure formation, in the case of plurallayers in which at least one of the second and subsequent layers has anirregular surface structure.

There were employed the same conditions as above, except that thedischarge was continued for 1.5 minutes, with the substrate at roomtemperature.

Then ZnO of the second layer was deposited by similar sputtering, with adischarge time of 15 minutes. Samples 16a, 16b, 16c, and 16d wereprepared by respectively maintaining the substrate at the roomtemperature, 100° C., 200° C., and 300° C. Table 12 summarizes theappearances and the result of SEM observation of these samples.

Light trapping can be attributed to light scattering in the metal layerin the case where the metal layer itself has the irregular structure,but to the scattering by aberration of the light phase between the peaksand valleys of the irregularities at the semiconductor surface and/orthe interface between the semiconductor and the transparent layer, inthe case where the metal layer is smooth and the transparent layer hasthe irregular surface structure. The pitch of irregularities ispreferably in a range from 3000 to 20,000 Å, more preferably from 4000to 15,000 Å, and the height of the irregularities is preferably in arange of 500 to 20,000 Å, more preferably from 700 to 10,000 Å. When thesemiconductor surface has an irregular structure similar to that of thetransparent layer, the scattering is facilitated by the phasedifference, so that the light trapping effect is enhanced.

For controlling the specific resistivity of the transparent layer, thereis preferably added a suitable impurity. In the transparent layer of thepresent invention, the resistance should preferably be suitably elevatedby the addition of an impurity, since the aforementioned conductiveoxides may have excessively low specific resistivities, and also forreducing the layer thickness. For example, an n-type semiconductor maybe doped with an acceptor-type impurity (such as Cu to ZnO or Al toSnO₂) in a suitable amount, for achieving intrinsic character andelevating the resistance.

In the case where transparent layer is composed of plural layers,suitable impurities may be added respectively to said layers, but asuitable resistance of the entire transparent layer may be obtained byintroducing the impurity in at least one layer.

Also, such an intrinsic transparent layer generally becomes moreresistant to the etching by acid or alkali. There are therefore obtainedadditional advantages in that it is less attacked by the chemicalsemployed in the patterning of the semiconductor layer or the ITO layerin the subsequent steps of manufacture of the photovoltaic devices, andthe durability of the photovoltaic device is improved under prolongeduse under high temperature and high humidity.

However, such an intrinsic state of the transparent layer is undesirableas it deteriorates the processing efficiency, if etching with an aqueoussolution of acid, alkali, or salt is employed for the formation of theirregular surface structure of the transparent layer. In such case, aphotovoltaic device of a suitable resistance, with high chemicalresistance and high durability, can be obtained by etching thetransparent layer without impurity introduction and by then laminatinganother transparent layer of intrinsic state. The impurity addition tothe transparent layer may be achieved by adding the desired impurity tothe evaporation source or the target as explained in Experiments 3 and4, or, in the case of sputtering, by placing a small piece of a materialcontaining the impurity on the target.

Embodiment 1

In this embodiment, there was prepared a photovoltaic device of the PINa-Si structure shown in FIG. 1 in which, however, the metal layer 102was absent. On Al plate 101 of 5×5 cm and a thickness of 1 mm, with apolished surface, ZnO layer 103 with an average thickness of 4000 Å wasdeposited, in the apparatus shown in FIG. 6 employing a ZnO targetcontaining 5% of Cu, with a substrate temperature of 250° C. The ZnOsurface showed an irregular structure.

The substrate bearing the thus-formed lower electrode was placed in acommercial capacitance-coupled high-frequency CVD apparatus (UlvacCHJ-3030), and the reaction chamber was evacuated roughly and thenfinely through a vacuum tub connected to a vacuum pump. The surfacetemperature of the structure was maintained at 250° C. by a temperaturecontrol mechanism. After sufficient evacuation, SiH₄ at 300 sccm, SiF₄at 4 sccm, PH₃ /H₂ (diluted to 1% with H₂) at 55 sccm, and H₂ at 40 sccmwere introduced from gas supply tubes, and the internal pressure of thereaction chamber was maintained at 1 Torr by the regulation of athrottle valve. When the pressure was stabilized, electric power of 200W was applied from a high-frequency power source, and the plasma wasmaintained for 5 minutes. Thus n-type a-Si layer 105 was formed on thetransparent layer 103. After evacuation of the reaction chamber again,SiH₄ at 300 sccm, SiF₄ at 4 sccm, and H₂ at 40 sccm were introduced fromthe gas supply tubes, and the internal pressure of the reaction chamberwas maintained at 1 Torr by the regulation of the throttle valve. Whenthe pressure was stabilized, electric power of 150 W was applied fromthe high-frequency power source, and the plasma was maintained for 40minutes. Thus i-type a-Si layer 106 was formed on the n-type a-Si layer105. After subsequent evacuation of the reaction chamber again, SiH₄ at50 sccm, BF₃ /H₃ (diluted to 1% with H₂) at 50 sccm, and H₂ at 500 sccmwere introduced from the gas supply pipes, and the internal pressure ofthe reaction chamber was maintained at 1 Torr by the regulation of thethrottle valve. When the pressure was stabilized, electric power of 300W was applied from the high-frequency power source, and the plasma wasmaintained for 2 minutes. Thus p-type μc-Si layer 107 was formed on thei-type a-Si layer 106. Subsequently the sample was taken out from theCVD apparatus, subjected to ITO deposition in a resistance-heated vacuumevaporation apparatus, then printing of a paste containing aqueousferric chloride solution for patterning the transparent electrode 108into desired shape, and screen printing of a silver paste for formingcurrent-collecting electrode 109, whereby a photovoltaic device wascompleted. 10 samples were prepared in this manner and subjected tomeasurement of photoelectric conversion efficiency under irradiation ofAM-1.5 light (100 mW/cm²), whereby an excellent efficiency of 9.5±0.2%was reproducibly obtained. Also, these photovoltaic devices, afterstanding for 1000 hours under the conditions of a temperature of 50° C.and a relative humidity of 90%, showed a conversion efficiency of9.2±0.5%, with almost no deterioration of the efficiency.

Embodiment 2

In this embodiment, a PIN a-SiGe photovoltaic device of theconfiguration shown in FIG. 1 was prepared. On surface-polishedstainless steel plate 101 of 5×5 cm and a thickness of 1 mm, there wasformed Cu layer 102 of a thickness of 1500 Å with a smooth surface byplating. Then Zn containing 1% of Cu was ion plated in an oxygenatmosphere with a substrate temperature of 350° C. to deposit a ZnOlayer of an average thickness of 1μ with an irregular surface structure.

10 samples were prepared by a subsequent process the same as inEmbodiment 1, except that the i-layer was composed of a-SiGe depositedby introducing Si₂ H₆ at 50 sccm, GeH₄ at 10 sccm, and H₂ at 300 sccm,maintaining the internal pressure of the reaction chamber at 1 Torr,applying an electrical power of 100 W, and maintaining the plasma for 10minutes. In measurement under AM-1.5 light (100 mW/cm²), these devicesshowed excellent conversion efficiency of 8.5±0.3% in a reproduciblemanner.

Embodiment 3

An apparatus shown in FIG. 7 was employed for forming the rearreflective layer in a continuous manner. In substrate feeding chamber603, there was placed roll 601 of cleaned stainless steel sheet 602 of awidth of 350 mm, a thickness of 0.2 mm and a length of 500 m. Thestainless steel sheet 602 was fed therefrom to takeup chamber 606,through metal layer deposition chamber 604 and transparent layerdeposition cheer 605. The sheet 602 could be heated to desiredtemperatures in respective deposition chambers by heaters 607, 608. Thechamber 604 was equipped with target 609 composed of Al of a purity of99.99%, for depositing Al onto the sheet 602 by DC magnetron sputtering.The chamber 605 was equipped with targets 610 of ZnO of a purity of99.5%, containing 0.5% of Cu, for depositing a ZnO layer by DC magnetronsputtering. There were provided four targets 610 in consideration of thedeposition rate and the desired film thickness.

The rear reflective layer was formed with this apparatus, at a sheetfeeding speed of 20 cm/min, and with a substrate temperature of 250° C.at the ZnO deposition station, achieved by the heater 608 alone. Argonwas supplied at a pressure of 1.5 mTorr, and a DC voltage of 500 V wasapplied to the cathodes. There were obtained a current of 6 A in thetarget 609 and a current of 4 A in each of the targets 610. On thetaken-up sheet there were obtained an Al layer of a thickness of 1600 Åand a ZnO layer of average thickness of 3800 Å, with a turbid surface.

An a-Si/a-SiGe tandem photovoltaic device of the configuration shown inFIG. 10 was formed thereon. In FIG. 10 there are shown substrate 701;metal layer 702; transparent layer 703; bottom cell 704; top cell 708;n-type a-Si layers 705, 709; p-type μc-Si layers 707, 711; i-type a-SiGelayer 706; and i-type a-Si layer 710. These semiconductor layers wereformed in continuous manner by a roll-to-roll film forming apparatus asdisclosed in U.S. Pat. No. 4,492,181. Transparent electrode 712 wasformed by a sputtering apparatus similar to that shown in FIG. 7. Thereis also provided a current-collecting electrode 713. After thepatterning of the transparent electrode and the formation of thecurrent-collecting electrodes, the sheet 602 was cut into pieces. Massproduction was achieved by conducting the entire process in a continuousmanner.

100 samples were prepared in this manner and evaluated under AM-1.5light (100 mW/cm²). There was obtained excellent conversion efficiencyof 11.2±0.2% in a reproducible manner. After standing for 1000 hoursunder conditions of 50° C. temperature and 90% relative humidity, theconversion efficiency was 10.8±0.6%, with almost no change. Also another100 samples, irradiated with light corresponding to AM-1.5 light for 600hours in the open circuit state, showed conversion efficiency of10.5±0.3%, indicating that the deterioration by light is also little.These results were based on the tandem structure which enabled effectiveabsorption of the long wavelength region and attained a higher outputvoltage, and also based on the reduced deterioration of thesemiconductor layer under light irradiation. Thus, in combination withthe effect of the rear reflective layer of the present invention, therecould be obtained a photovoltaic device of high reliability and a highconversion efficiency.

Embodiment 4

A rear reflective layer was formed in the same manner as in Embodiment1, except that a surface polished Cu plate was employed as thesubstrate. On the thus-treated substrate and on a substrate without ZnOdeposition, Cu and In were deposited by sputtering, with respectivethicknesses of 0.2 and 0.4μ. The samples were then transferred to aquartz glass bell jar and heated to 400° C. therein, and hydrogenselenide (H₂ Se) diluted to 10% with hydrogen was supplied therein toform a thin film of CuInSe₂ (CIS). Then a CdS layer of a thickness of0.1μ was formed thereon by sputtering, and a p/n junction was formed byannealing at 250° C. A transparent electrode and current-collectingelectrodes were then formed thereon in the same manner as in Embodiment1.

In the evaluation of the thus-obtained photovoltaic devices underirradiation of AM-1.5 light (100 mW/cm²), the device with the ZnO layershowed excellent conversion efficiency of 9.5% while the device withoutZnO showed inferior efficiency of 7.3% only. These results indicate thatthe present invention is effective also for semiconductors other thana-Si.

Embodiment 5

In the present embodiment, there was prepared a PIN a-Si photovoltaicdevice of the configuration shown in FIG. 2, however without metal layer102. On surface-polished Al plate 101 of 5×5 cm and a thickness of 1 mm,ZnO layer 103a was deposited to an average thickness of 4000 Å, in theapparatus shown in FIG. 6 employing a ZnO target, with a substratetemperature of 300° C. The ZnO surface showed an irregular structure.Then, after the substrate temperature was lowered to room temperature,ZnO layer 103b was deposited to a thickness of 500 Å.

On the thus-obtained rear reflective layer, PIN a-Si semiconductor layer104, transparent electrode 108, and current-collecting electrode 109were formed in the same manner as in Embodiment 1 to complete thephotovoltaic device. 10 samples were prepared in this manner andevaluated under irradiation of AM-1.5 light (100 mW/cm²). Excellentphotoelectric conversion efficiency of 9.7±0.2% was reproduciblyobtained. Also, after standing for 1000 hours under the conditions of atemperature of 50° C. and a relative humidity of 90%, these devicesshowed conversion efficiency of 9.4±0.5%, with almost no deterioration.

Embodiment 6

In the present embodiment there was prepared a PIN a-SiGe photovoltaicdevice of the configuration shown in FIG. 2. On surface-polishedstainless steel plate 101 of 5×5 cm and a thickness of 1 mm, Cu layer102 with a smooth surface was formed to a thickness of 1500 Å byplating. Subsequently, Zn was ion plated in an oxygen atmosphere with asubstrate temperature of 350° C. to deposit a ZnO layer of an averagethickness of 1μ, with an irregular surface structure. Then the substratewas immersed in 10% aqueous acetic acid solution for 45 seconds, thendried for 20 minutes in a thermostat chamber of 80° C., and subjected tothe deposition of a ZnO layer of a thickness of 700 Å by theabove-mentioned ion plating, with the substrate at room temperature.

10 samples were prepared in the same subsequent process as in Embodiment1, except that an a-SiGe layer, deposited by introducing Si₂ H₆ at 50sccm, GeH₄ at 10 sccm, and H₂ at 300 sccm, maintaining the internalpressure of the reaction chamber at 1 Torr, applying electric power of100 W and maintaining the plasma for 10 minutes, was employed as thei-type layer. In evaluation under irradiation of AM-1.5 light (100mW/cm²), these devices showed excellent conversion efficiency of8.7±0.3% in a reproducible manner.

Embodiment 7

A rear reflective layer was continuously prepared by the apparatus shownin FIG. 8. In feed chamber 603, roll 601 of a cleaned stainless steelsheet of a width of 350 mm, a thickness of 0.2 mm, and a length of 500 mwas placed. The stainless steel sheet 602 was fed therefrom to take-upchamber 606 through metal layer deposition cheer 604, first layerdeposition cheer 605a and second layer deposition cheer 605b. The sheet602 could be heated to desired temperatures in said chambers byrespective heaters 607, 608a, and 608b. The deposition cheer 604 wasequipped with target 609 of Al of a purity of 99.99% for depositing anAl layer on the sheet 602 by DC magnetron sputtering. The cheers 605a,605b were equipped with targets 610a, 610b of ZnO of a purity of 99.9%for depositing ZnO layers in succession by DC magnetron sputtering.There were provided four targets 610a while the target 610b was of ahalf width, in consideration of the deposition rate and the desired filmthickness.

The rear reflective layer was formed with this apparatus in thefollowing manner. The sheet was fed with a speed of 20 cm/min, and washeated to 250° C. at the ZnO deposition by the heater 608a only, whilethe heater 608b was not used. Argon was supplied at a pressure of 1.5mTorr, and a DC voltage of 500 V was supplied to the cathodes, wherebythere were obtained currents of 6 A in the target 609, 4 A in each ofthe targets 610a, and 2 A in the target 610b. On the taken-up sheet andAl layer and a thickness of 4300 Å in the total of two layers, and theZnO layer had a turbid surface.

On the thus-obtained sheet there was formed a tandem a-Si/a-SiGephotovoltaic device of the configuration shown in FIG. 11, in whichthere are shown substrate 701; metal layer 702; transparent layer 703;bottom cell 704; top cell 708; first layer 703a of ZnO; second layer703b of ZnO; n-type a-Si layers 705, 709; p-type μc-Si layers 707, 711;i-type a-SiGe layer 706; and i-type a-Si layer 710. These semiconductorlayers were prepared continuously in a roll-to-roll film formingapparatus as disclosed in U.S. Pat. No. 4,492,181. Transparent electrode712 was deposited by a sputtering apparatus similar to the one shown inFIG. 8. Current-collecting electrode 713 was formed thereon. After thepatterning of the transparent electrode and the formation of thecurrent-collecting electrode, the sheet 602 was cut into pieces. Massproduction was thus achieved, by effecting the entire process in acontinuous manner.

100 samples were prepared in this manner and evaluated under irradiationof AM-1.5 light (100 mW/cm²). Excellent photoelectric conversionefficiency of 11.5±0.2% was obtained in a reproducible manner. Also,after standing for 1000 hours under conditions of a temperature of 50°C. and a relative humidity of 90%, these devices showed conversionefficiency of 11.0±0.6%, with almost no deterioration. Another 100samples prepared in this manner, irradiated by light corresponding toAM-1. light for 600 hours in the open circuit state, showed anefficiency of 10.7±0.3%, indicating that the deterioration by light isalso limited. These results were due to the tandem structure whichenabled effective absorption of the longer wavelength region of thelight, thereby providing a higher output voltage, and also to thereduced deterioration of the semiconductor layer under lightirradiation. Thus, in combination with the effect of the rear reflectivelayer of the present invention, a photovoltaic device of high conversionefficiency and-high reliability could be obtained.

Embodiment 8

A rear reflective layer was formed in the same manner as in Embodiment5, except for the use of a surface-polished Cu plate as the substrate.Then, on the thus-processed substrate and on a substrate withoutdeposition of a second layer of ZnO, there were deposited Cu and In withrespective thicknesses of 0.2 and 0.4μ by sputtering. Subsequently, thesamples were transferred to a quartz glass bell gar, heated to 400° C.,and supplied with hydrogen selenide (H₂ Se) diluted to 10% with hydrogento form a thin film of CuInSe₂ (CIS). Then a CdS layer of a thickness of0.1μ was formed thereon by sputtering, and annealing was conducted at250° C. to form a p/n junction. Then a transparent electrode and acurrent-collecting electrode were formed thereon in the same manner asin Embodiment 5.

In evaluation of the thus-obtained photovoltaic devices underirradiation of AM-1.5 light (100 mW/cm²), the devices with two ZnOlayers showed high conversion efficiency of 9.6% while the devices withone ZnO layer also showed high efficiency of 9.5%. However, in themeasurement of I-V characteristics, devices in which the shuntresistance per unit area does not exceed 200 Ωcm were found in 4% of allthe devices with two layers and in 28% of all the devices with onelayer. Consequently, the reliability was higher in the two-layeredstructure.

Embodiment 9

In the present embodiment, there was prepared a PIN a-Si photovoltaicdevice of the configuration shown in FIG. 3, however without the metallayer 102. On a surface-polished Al plate 101 of 5×5 cm and a thicknessof 1 mm, there was deposited ZnO layer 103c of a thickness of 1000 Å, inthe apparatus shown in FIG. 6 employing a ZnO target, with the substrateat room temperature. Then, ZnO layer 103d of an average thickness of3000 Å was deposited thereon, with a substrate temperature of 300° C.,whereby the ZnO surface showed an irregular structure.

On the thus-obtained rear reflective layer, there were formed PIN a-Sisemiconductor layer 104, transparent electrode 108 andcurrent-collecting electrode 109 in the same manner as in Embodiment 1to complete a photovoltaic device. 10 samples were prepared in thismanner and evaluated under irradiation of AM-1.5 light (100 mW/cm²).There was obtained excellent photoelectric conversion efficiency of9.6±0.2% in a reproducible manner. Also after standing for 1000 hoursunder conditions of a temperature of 50° C. and a relative humidity of90%, these devices showed conversion efficiency of 9.3±0.5%, with almostno deterioration.

Embodiment 10

In the present embodiment, there was prepared a PIN a-SiGe photovoltaicdevice of the configuration shown in FIG. 3. On surface-polishedstainless steel plate 101 of 5×5 cm and a thickness of 1 mm, Cu layer102 of a thickness of 1500 Å, with a smooth surface, was formed byplating. Then Zn was ion-plated in an oxygen atmosphere with thesubstrate at room temperature, thus depositing ZnO layer 103c of athickness of 800 Å.

Subsequently, ZnO layer 103d of a thickness of 1μ, with an irregularsurface structure, was deposited by effecting ion plating again, with asubstrate temperature of 250° C.

10 samples were prepared by a subsequent process the same as inEmbodiment 9, except that the i-type was replaced by an a-SiGe layerwhich was deposited by introducing Si₂ H₆ at 50 sccm, GeH₄ at 10 sccm,and H₂ at 300 sccm, maintaining the internal pressure of the reactionchamber at 1 Torr, applying electric power of 100 W and maintaining theplasma for 10 minutes. In evaluation of these devices under irradiationof AM-1.5 light (100 mW/cm²), excellent photoelectric conversionefficiency of 8.6±0.4% could be obtained in a reproducible manner.

Embodiment 11

A rear reflective layer was formed in a continuous manner with anapparatus shown in FIG. 9. In feed chamber 603 there was placed roll 601of a cleaned stainless steel sheet of a width of 350 mm, a thickness of0.2 mm, and a length of 500 m. Said stainless steel sheet 602 was fedtherefrom to take-up chamber 606, through metal layer deposition chamber604, first layer deposition chamber 605c, and second layer depositionchamber 605d. The sheet 602 could be heated to desired temperatures atrespective deposition chambers, by means of heaters 607, 608c, and 608d.The deposition chamber 604 was equipped with target 609 of Al of apurity of 99.99%, for depositing an A1 layer on the sheet 602 by DCmagnetron sputtering. The deposition chambers 605c, 605d were equippedwith targets 610c, 610d of ZnO of a purity of 99.9%, for depositing ZnOlayers in succession by DC magnetron sputtering. The target 610c was ofa half width, while there were provided four targets 610d, inconsideration of the deposition rate and the desired film thicknesses.

This apparatus was employed in the formation of the rear reflectivelayer in the following manner. The sheet was fed with a speed of 20cm/min, and was maintained at 250° C. at the ZnO deposition chamber 605dby the heater 608d only. The heater 608c was not employed, and thesubstrate was maintained at room temperature. Argon was supplied at apressure of 1.5 mTorr, and DC voltage of 500 V was applied to thecathodes, whereby there were obtained currents of 6 A in the target 609,2 A in the target 610c, and 4 A in each of the targets 610d. On thetaken-up sheet, the Al layer had a thickness of 1600 Å, while the ZnOlayer had an average thickness of 4400 Å in the total of the two layersand had a turbid surface.

A tandem a-Si/a-SiGe photovoltaic device of the configuration shown inFIG. 12 was formed thereon. In FIG. 12 there are shown substrate 701;metal layer 702; transparent layer 703; bottom cell 704; top cell 708;first layer 703c; second layer 703d; n-type a-Si layers 705, 709; p-typeμc-Si layers 707, 711; i-type a-SiGe layer 706; and i-type a-Si layer710. These semiconductor layers were prepared in a continuous manner,with a roll-to-roll film forming apparatus as disclosed in U.S. Pat. No.4,492,181. Transparent electrode 712 was deposited in a sputteringapparatus similar to the one shown FIG. 9. Current-collecting electrode713 was formed thereon. After the patterning of the transparentelectrode and the formation of the current-collecting electrode, thesheet 602 was cut into pieces. Mass production thus could be achieved byeffecting the entire process in continuous manner.

100 samples were prepared in the manner and evaluated under irradiationof AM-1.5 light (100 mW/cm²). Excellent photoelectric conversionefficiency of 11.3±0.2% was obtained in a reproducible manner. Also,after standing for 1000 hours under conditions of a temperature of 50°C. and a relative humidity of 90%, these devices showed conversionefficiency of 11.1±0.6%, with almost no deterioration. Also, another 100samples prepared in this method showed, after irradiation with lightcorresponding to AM-1.5 light for 600 hours in the open circuit state,conversion efficiency of 10.7±0.3%, indicating that the deterioration bylight is also limited. The results were due to the tandem structurewhich enabled efficient absorption of the longer wavelength region ofthe light, thereby elevating the output voltage, and also to the reduceddeterioration of the thin film semiconductor layer under the lightirradiation. Thus, in combination with the effect of the rear reflectivelayer of the present invention, there could be obtained a photovoltaicdevice of a high conversion efficiency and high reliability.

Embodiment 12

A rear reflective layer was prepared in the same manner as in Embodiment9, except for the use of a surface-polished Cu plate as the substrate.Then, on the thus-treated substrate and on a substrate lacking thedeposition of the second ZnO layer, Cu and In were deposited withrespective thicknesses of 0.2 and 0.4μ by sputtering. Subsequently, thesamples were transferred to a quartz glass bell jar, heated to 400° C.and supplied with hydrogen selenide (H₂ Se) diluted to 10% withhydrogen, for forming a thin film of CuInSe₂ (CIS). Then a CdS layer wasdeposited thereon with a thickness of 0.1μ by sputtering, and annealingwas conducted at 250° C. to form a p/n junction. A transparent electrodeand a current-collecting electrode were formed thereon in the samemanner as in Embodiment 9.

In evaluation of these photovoltaic devices under irradiation of AM-1.5light (100 mW/cm²), the device with two ZnO layers showed excellentconversion efficiency of 9.6% while the device with a smooth ZnO layeronly showed inferior conversion efficiency of 8.3%. These resultsindicate that the present invention is effective also withsemiconductors other than a-Si.

Embodiment 13

This embodiment shows that integration of a secondary battery, such as anickel-cadmium (NiCd) battery, can be extremely facilitated byintegrating said battery with a solar cell produced by the method of thepresent invention.

A tandem a-Si/A-SiGe photovoltaic device was prepared in the same manneras in Embodiment 3, except that the roll of stainless steel sheetsubstrate was replaced by a roll of cold-rolled steel sheet of JISG3141with a width of 350 mm and a thickness of 0.015 mm, provided with nickelplating of a thickness of 5μ. The transparent electrode was patterned ina size of 58×100 mm, and, after the formation of the current-collectingelectrode, the device was cut into a size of 70×110 mm. FIG. 13 shows abattery employing the thus-prepared photovoltaic device as thecontainer. FIG. 13A is an external view of said battery.

The photovoltaic device as explained above is incorporated on thecontainer 1501, and a sturdy bottom plate is provided for withstandingthe pressure of the gas generated inside. Said bottom plate does nothave the photovoltaic device and serves as the cathode terminal. FIG.13B shows the internal structure of the battery. Inside the batterythere are wound cathode plate 1504 and anode plate 1505, separated byseparator 1506. Said plates 1504, 1505 are composed of sintered Ni-Cdalloy, while the separator 1506 is composed of nylon non-woven clothimpregnated with electrolyte solution of potassium hydroxide. Thecathode plate 1504 is connected to the container 1501, while the anodeplate 1505 is connected to anode terminal 1502. The interior is sealedwith plastic lid 1507, equipped with packing 1508, for preventingelectrolyte leakage. However, the lid 1507 is provided with safety valve1509, in order to prevent accidents resulting from a rapid pressureincrease at rapid charging or discharging.

Grid electrode 1513 on the surface of the photovoltaic device isconnected to a lead wire, which is connected to the anode terminal 1502through diode 1503 for preventing reverse current. In order to protectthe surface of the photovoltaic device, a cylindrical heat-shrinkablesheet is placed on the container and heated with hot air, therebycovering the battery except for the anode terminal 1502 and the bottomplate. FIG. 14 shows the equivalent circuit of the above-explainedconnections. The battery 1510 is connected, by the cathode terminal(container) 1501 and the anode terminal 1502, to external load 1512.

When the photovoltaic device 1511 is irradiated with light, there isgenerated a photovoltaic force of about 1.6 V, which is applied to thebattery 1510. As the voltage of the battery is about 1.2 V at maximum,diode 1503 is biased in the forward direction, and the battery 1510 ischarged by the photovoltaic device 1511.

However, when the photovoltaic device 1511 is not irradiated with light,the diode 1503 is biased in the reverse direction, so that current isnot supplied unnecessarily from the battery 1510. Also in the case wherethe battery is charged in an ordinary charger, the charging can beachieved in the usual manner, without the waste of the current flowingin the photovoltaic device 1511, by the function of said diode 1503.Consequently, the battery of the present embodiment, equipped with thephotovoltaic device, can be charged by light, by being taken out fromthe battery case after use and being left in a place with strong lightirradiation such as at a window, or even within the battery case if ithas a transparent cover. This battery is particularly convenient for useoutdoors, since it does not require a particular charger. It can also becharged with an ordinary charger, in the case where rapid charging isrequired. It can also be formed in the same manner as the ordinarybatteries, such as UM1, UM2, or tank-shaped batteries. It can thereforebe used in various electric appliances and has a smart appearance.

Two such batteries are loaded in a portable flashlight, and, when theyare exhausted, are removed therefrom and allowed to stand aside a sunnywindow for charging. After a fine day, they are sufficiently charged andcan be used again in the flashlight.

Embodiment 14

In an artificial satellite, there is generally employed a compoundsemiconductor crystalline type solar cell, such as of InP, which has alarge output per unit area and is resistant to radiation. However, sincesuch solar cell is composed of wafers, it has to be fixed on a panel. Atthe satellite launching, the panel has to be compacted and requires acomplex fold-unfold mechanism for a plurality of panels. For thisreason, even if the output per unit area is high, the output per unitweight has heretofore had to be considerably low.

The present embodiment provides a satellite power source with a largeoutput per unit weight with a simple mechanism, through the use of thesolar cell, as an example of the photovoltaic device produced by themethod of the present invention.

Referring to FIG. 15, a tandem a-Si/A-SiGe photovoltaic device wasprepared in the same manner as in Embodiment 13, except that the roll ofstainless steel substrate was replaced by a roll of an aluminum sheet ofJIS2219 (including copper, manganese, etc.) of a width of 350 mm and athickness of 0.15 mm. Transparent electrode 1512 was patterned into asize of 105×320 mm, and, after the formation of current-collectingelectrode 1513, the sheet was cut into pieces. An end of the longer sideof each photovoltaic device was polished with a grinder to expose thesubstrate surface. Then the devices were serially connected as shown inFIG. 15. The devices 1701 and 1702 were connected, with a mutualdistance of about 5 mm, from the rear side by means of insulating film1703 composed, for example, of polystyrene, polyimide, cellulosetriacetate or trifluorethylene, and a current-collecting electrode 1513of the device 1701 and the exposed substrate portion 1704 of the device1702 were connected by heat pressing with copper sheet 1705 utilizingcopper ink or silver ink. For avoiding short-circuiting between thecopper sheet 1705 and the substrate of the device 1701, polyimide film1706 is applied on the edge portion. Protective polyester film 1701 wasadhered thereover.

200 photovoltaic devices were serially connected to form a solar cell ofa length of about 20 m, as an application of the photovoltaic device.Said solar satellite cell can be constructed as shown in FIG. 16. Themain body 1901 of the satellite is provided with rotatable shaft 1902,on which sheet-shaped photovoltaic devices 1903, 1904, etc. are wound.1903 indicates a completely extended state of the device while 1904indicates a half-wound state. The current generated at the end of thesheet-shaped photovoltaic device is supplied to the main body of thesatellite, by means of a cable (not shown) that can be wound togetherwith the photovoltaic device. Said sheet-shaped photovoltaic devices1903, 1904 and said cable can be extended or wound by a driving system(not shown).

This system is used in the following manner. At the launching of thesatellite, the sheet-shaped photovoltaic devices 1903, 1904 aremaintained in the wound state. After the satellite is placed on theorbit, it is made to rotate slowly about the rotational axis directedtoward the sun. At the same time the sheet-shaped photovoltaic devicesare slowly extended, whereby they are stretched radially by centrifugalforce and start power generation. In the case of an orbit change,positional change, or recovery of the satellite, the sheet-shapedphotovoltaic devices are wound, for example, by a motor. Thereafter theycan be extended again, when required, to re-start the power generation.A system with six sheets of the photovoltaic devices has a maximumoutput of 5 kW with a total weight of ca. 30 kg including the drivingsystem, so that a large output per unit weight can be achieved.

Embodiment 15

The present embodiment provides a roofing material obtained by undulatedforming of the photovoltaic devices produced by the method of thepresent invention. A photovoltaic device prepared according to themethod of Embodiment 3 was cut into a length of 100 mm and a width of900 mm, then the obtained sheets were individually pressed intoundulated form and adhered to corrugated substrates of a length of 1800mm and a width of 900 mm, composed, for example, of polyvinyl chlorideresin or polyester resin. FIG. 17 shows the details of connection of thephotovoltaic devices. Devices 2001 and 2002 are adhered, with a mutualdistance of 10 mm, on corrugated substrate 2003.

A grid electrode of the device 2001 and an exposed substrate portion ofthe device 2002 are connected by copper sheet 2005. There is providedinsulating film 2006 for preventing short-circuiting composed, forexample, of polyimide resin, polyvinyl alcohol resin, or polystyreneresin. Nail holes 2007 are provided in advance in the corrugatedsubstrate 2003, for fixing the roofing material. For avoidingshort-circuiting, the copper sheet 2005 is provided with holes 2008larger than said nail holes. Said nail holes are provided only in thenecessary serial connecting part. PVA resin layer 2009 and fluorinatedresin layer 2010 are superposed thereon and adhered by pressurizedheating to obtain an integral roofing material. FIG. 18 shows a roofobtained with said roofing materials (number of undulations is eachintegral roofing material 2102 being reduced for the purpose ofsimplicity). FIG. 17 shows the details of each photovoltaic device 2101of the roofing material 2102. At the serial connecting portions 2103,2104, it is connected to the neighboring photovoltaic devices.

Serial connecting portion 2103 has nail holes but 2104 does not havenail holes. It is fixed, with overlapping of an undulation with anotherroofing material 2105. The photovoltaic device is not present at theleft-hand end of the roofing material 2102. Since the corrugatedsubstrate is transparent, it does not block the light even if it isplaced on the adjacent roofing material 2105. The output terminal at theleft-hand end of the roofing material 2102 is connected in advance withthe output terminal at the right-hand end of the material 2105 beforethe material 2102 is placed on the portion 2106, and the connecting partis sealed, for example, with anticorrosive paint and is so positioned asnot to be exposed to the exterior. In this manner, the serial connectionis completed simultaneously with the installation of plural roofingmaterials.

If the serial connection is not conducted, the output terminal of eachdevice can be connected to a cable which can be guided under convexportion 2108 for supplying the output to the exterior.

A serial connection of four of such roofing materials, when connected inparallel in 8 sets and installed on a roof inclined by 30° toward thesouth, provides an output of about 5 kW in the summer daytime,sufficient as the electric power source for an ordinary household.

Embodiment 16

The present embodiment provides a photovoltaic device for an automobile,usable for driving a ventilation fan or preventing the discharge ofbattery. The photovoltaic devices are starting to be employed inautomobiles, but they are generally installed on the sun roof or on thestabilizer fin, in order not to affect the automobile design. In fact, asuitable place for installation is difficult to find in automobiles ofordinary specifications. The engine hood or the roof is adequate forreceiving the sunlight, but the installation in these locations mayeasily affect the design of the automobile. Also the front, rear orlateral face of the automobile is susceptible to damage by contact. Onthe other hand, the rear quarter pillars of the automobile can provide asuitable area, are relatively free from damaging effects and allow easymatching with the automobile design.

The present embodiment utilizes the feature of the photovoltaic deviceof the present invention, constructed on a sheet-shaped metal substrate,and provides a photovoltaic device having a curved form that can beintegrated with the automobile body when installed on the rear quarterpillars.

A photovoltaic device was prepared in the same manner as in Embodiment14, and individual photovoltaic pieces were obtained by effecting thepatterning of the transparent electrode, the formation of thecurrent-collecting electrode and the cutting according to the automobiledesign.

The thus-obtained pieces were serially connected according to the methodof Embodiment 15. For use in an automobile equipped with a battery of 12V, FIG. 19 shows a configuration consisting of 10 solar cell pieces.Pieces 2201 in the upper portion of the pillar are made longer becauseof the smaller width, while those 2202 in the lower portion are madeshorter because of the large width, in such a manner that all the pieceshave substantially the same area. Also, the current-collectingelectrodes 2203 are provided denser for the longer pieces, in order tosuppress the power loss resulting from the resistance in the transparentelectrode.

The color of the photovoltaic devices is an important factor in thedesign, but the thickness of the transparent electrode can be regulatedso as to match the color of other parts of the automobile. An ITOtransparent electrode with an ordinary thickness of 650 Å provides apurple appearance. Said color becomes yellow-green in a thickness rangeof 450 to 500 Å, brownish in 500 to 600 Å, purple in 600 to 700 Å, andmore bluish in 700 to 800 Å. The output of the photovoltaic device issomewhat lowered when the thickness of the ITO layer is deviated fromthe standard value, but the loss in efficiency can be minimized in thecase of a tandem call, by adjusting the spectral sensitivities of thetop and bottom cells, by forming the top cell thinner for a thinner ITOlayer because the spectral sensitivity is shifted toward the shorterwavelength, and forming the top cell thicker for a thicker ITO layer.

Photovoltaic modules 2302 of the present embodiment were installed onthe left and right rear quarter pillars of 4-door sedan automobile 2301(coated blue) shown in FIG. 20. FIG. 19 showwos the module for use onthe left-side pillar. Said devices are incorporated in a circuit shownin FIG. 21, in which the left-side device 2401 and the right-side device2402 are respectively connected to battery 2405 through diodes 2403,2404. Ventilation fan 2406 is activated when the inside temperature ishigh because of the strong sunlight. Switch 2407 for the ventilation fan2406 is turned on only when at least either of current sensors 2409,2410 for detecting the output currents from the left-side and right-sidephotovoltaic devices provides a high-level signal and temperature sensor2408 provides a high-level signal.

Such photovoltaic modules can reduce the inside temperature, whicheasily reaches 80° C. on a fine summer day, by about 30° C., and alsoavoid the exhaustion of the battery even when it is left unused for aweek in mid-winter. Such configuration is also usable in snowy areas,because the snow accumulated on the rear quarter pillars is quicklymelted by sunlight.

                  TABLE 1                                                         ______________________________________                                                                              Con-                                                      Metal     Transparent                                                                             version                                 Sample                                                                              Substrate   Layer     layer     efficiency                              ______________________________________                                        1a    stainless steel                                                                           A1        ZnO (4000 Å)                                                                        9.8%                                          plate       (1500 Å)                                                                            irregular                                               (SUS 430)   smooth    structure                                                           surface                                                     1b    stainless steel                                                                           A1,       ZnO       8.0%                                          plate       irregular                                                   1c    stainless steel                                                                           A1, smooth                                                                              ZnO, smooth                                                                             8.2%                                          plate                                                                   1d    stainless steel                                                                           --        --        6.7%                                          plate                                                                   1e    aluminum plate                                                                            --        ZnO, irregular                                                                          9.6%                                    ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                                              Con-                                                      Metal     Transparent                                                                             version                                 Sample                                                                              Substrate   Layer     layer     efficiency                              ______________________________________                                        2a    stainless steel                                                                           silver,   ZnO, irregular                                                                          10.0%                                         plate (SUS 430)                                                                           smooth                                                      2b    stainless steel                                                                           silver,   ZnO, irregular                                                                           2.7%                                         plate       irregular                                                   ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                                                              Con-                                                      Metal     Transparent                                                                             version                                 Sample                                                                              Substrate   Layer     layer     efficiency                              ______________________________________                                        3a    stainless steel                                                                           A1        ZnO (non- 1.5%                                          plate (SUS 430)                                                                           (1500 Å)                                                                            doped),                                                             smooth    irregular                                         3b    stainless steel                                                                           A1, smooth                                                                              ZnO (Cu   7.4%                                          plate                 doped),                                                                       irregular                                         ______________________________________                                    

                                      TABLE 4                                     __________________________________________________________________________                          Transparent layer  Conversion                           Sample                                                                             Substrate                                                                              Metal layer                                                                           1st layer  2nd layer                                                                             efficiency                           __________________________________________________________________________    6a   stainless steel                                                                        A1 (1500 Å),                                                                      ZnO (4000 Å)                                                                         ZnO (500 Å)                                                                       10.0%                                     plate (SUS 430)                                                                        irregularity                                                                          pitch 4000-9000 Å                                                 1000 Å                                                                            height 2000-4000 Å                                  6b   stainless steel                                                                        A1 (1500 Å,                                                                       ZnO (4000 Å)                                                                         ZnO (500 Å)                                                                       8.5%                                      plate (SUS 430)                                                                        irregularity                                                                          pitch 4000-9000 Å                                                 20000 Å                                                                           height 2000-4000 Å                                  6c   stainless steel                                                                        A1 (1500 Å,                                                                       ZnO (4000 Å)                                                                         --      9.7%                                      plate (SUS 430)                                                                        irregularity                                                                          pitch 4000-9000 Å                                                 1000 Å                                                                            height 2000-4000 Å                                  6d   stainless steel                                                                        A1 (1500 Å,                                                                       ZnO (4500 Å)                                                                         --      9.6%                                      plate (SUS 430)                                                                        irregularity                                                                          pitch 4000-9000 Å                                                 1000 Å                                                                            height 2000-4000 Å                                  6e   stainless steel                                                                        A1 (1500 Å,                                                                       ZnO (4000 Å)                                                                         --      8.2%                                      plate (SUS 430)                                                                        irregularity                                                                          pitch 2500 Å max.                                                 1000 Å                                                                            height 500 Å max.                                   6f   stainless steel                                                                        A1 (1500 Å,                                                                       ZnO (4500 Å)                                                                         --      8.0                                       plate (SUS 430)                                                                        irregularity                                                                          pitch 2500 Å max.                                                 1000 Å                                                                            height 500 Å max.                                   6g   aluminum --      ZnO (4000 Å)                                                                         ZnO (500 Å)                                                                       9.8%                                                       pitch 4000-9000 Å                                                         height 2000-4000 Å                                  __________________________________________________________________________

                                      TABLE 5                                     __________________________________________________________________________                          Transparent layer  Conversion                           Sample                                                                             Substrate                                                                              Metal layer                                                                           1st layer  2nd layer                                                                             efficiency                           __________________________________________________________________________    7a   stainless steel                                                                        A1 (1500 Å),                                                                      ZnO (10000 Å)                                                                        ZnO (500 Å)                                                                       9.9%                                      plate (SUS 430)                                                                        irregularity                                                                          pitch 6000-10000 Å                                                1000 Å                                                                            height 3000-8000 Å                                  7b   stainless steel                                                                        A1 (1500 Å),                                                                      ZnO (25000 Å)                                                                        ZnO (500 Å)                                                                       9.7%                                      plate (SUS 430)                                                                        irregularity                                                                          pitch 6000-10000 Å                                                1000 Å                                                                            height 3000-8000 Å                                  7c   stainless steel                                                                        A1 (1500 Å),                                                                      ZnO (25000 Å)                                                                        ZnO (500 Å)                                                                       9.4%                                      plate (SUS 430)                                                                        irregularity                                                                          pitch 9000-14000 Å                                                1000 Å                                                                            height 5000-10000 Å                                 7d   stainless steel                                                                        A1 (1500 Å),                                                                      ZnO (25000 Å)                                                                        ZnO (500 Å)                                                                       8.6%                                      plate (SUS 430)                                                                        irregularity                                                                          pitch 18000-28000 Å                                               1000 Å                                                                            height 8000-15000 Å                                 __________________________________________________________________________

                                      TABLE 6                                     __________________________________________________________________________                          Transparent layer     Conversion                        Sample                                                                             Substrate                                                                              Metal layer                                                                           1st layer  2nd layer  efficiency                        __________________________________________________________________________    9a   stainless steel                                                                        A1 (1500 Å)                                                                       ZnO (1000 Å)                                                                         ZnO (3000 Å)                                                                         9.9%                                   plate (SUS 430)                                                                        irregularity                                                                          irregularity                                                                             pitch 4000-8000 Å                                      pitch 1000 Å                                                                      pitch 1000 Å                                                                         height 2000-3000 Å                       9b   stainless steel                                                                        A1 (1500 Å)                                                                       ZnO (1000 Å)                                                                         ZnO (3000 Å)                                                                         8.7%                                   plate (SUS 430)                                                                        irregularity                                                                          irregularity                                                          pitch 2000 Å                                                                      pitch 1500 Å                                        9c   stainless steel                                                                        A1 (1500 Å)                                                                       ZnO (1000 Å)                                                                         --         8.3%                                   plate (SUS 430)                                                                        irregularity                                                                          irregularity                                                          pitch 1000 Å                                                                      pitch 1000 Å                                        9d   stainless steel                                                                        A1 (1500 Å)                                                                       ZnO (4000 Å)                                                                         --         8.2%                                   plate (SUS 430)                                                                        irregularity                                                                          pitch 2500 Å max.                                                 pitch 1000 Å                                                                      height 500 Å max.                                   9e   stainless steel                                                                        A1 (1500 Å)                                                                       ZnO (4000 Å)                                                                         --         9.7%                                   plate (SUS 430)                                                                        irregularity                                                                          pitch 4000-9000 Å                                                 pitch 1000 Å                                                                      height 2000-4000 Å                                  9f   aluminum plate                                                                         --      ZnO (1000 Å)                                                                         ZnO (3000 Å)                                                                         9.7%                                                    irregularity                                                                             pitch 4000-8000 Å                                              pitch 1000 Å                                                                         height 2000-3000 Å                       __________________________________________________________________________

                  TABLE 7                                                         ______________________________________                                                                   Transparent                                                                            Conversion                                Sample                                                                              Substrate Metal Layer                                                                              layer    efficiency                                ______________________________________                                        10a   stainless silver,    two-layered,                                                                           10.3%                                           steel plate                                                                             smooth     with                                                     (SUS 430)            irregular                                                                     structure                                                                     in the                                                                        2nd ZnO                                                                       layer                                              10b   stainless silver,    two-layered                                                                            2.9%                                            steel plate                                                                             irregular  ZnO,                                                                          irregular                                                                     structure                                          ______________________________________                                    

                                      TABLE 8                                     __________________________________________________________________________                          Transparent layer    Conversion                         Sample                                                                             Substrate                                                                              Metal layer                                                                           1st layer                                                                              2nd layer   efficiency                         __________________________________________________________________________    11a  stainless steel                                                                        A1 (1500 Å)                                                                       SnO.sub.2 (1500 Å)                                                                 ZnO (10000 Å)                                                                         9.5%                                    plate (SUS 430)                                                                        irregularity                                                                          irregularity                                                                           pitch 6000-9000 Å                                        pitch 1000 Å                                                                      pitch 1000 Å                                                                       height 4000-7000 Å                         11b  stainless steel                                                                        A1 (1500 Å)                                                                       SnO.sub.2 (1500 Å)                                                                 ZnO (10000 Å)                                                                         9.2%                                    plate (SUS 430)                                                                        irregularity                                                                          irregularity                                                                           pitch 8000-15000 Å                                       pitch 1000 Å                                                                      pitch 1000 Å                                                                       height 8000-10000 Å                        11c  stainless steel                                                                        A1 (1500 Å)                                                                       SnO.sub.2 (1500 Å)                                                                 ZnO (25000 Å)                                                                         9.3%                                    plate (SUS 430)                                                                        irregularity                                                                          irregularity                                                                           pitch 6000-9000 Å                                        pitch 1000 Å                                                                      pitch 1000 Å                                                                       height 4000-7000 Å                         11d  stainless steel                                                                        A1 (1500 Å)                                                                       SnO.sub.2 (1500 Å)                                                                 ZnO (25000 Å)                                                                         9.1%                                    plate (SUS 430)                                                                        irregularity                                                                          irregularity                                                                           pitch 8000-15000 Å                                       pitch 1000 Å                                                                      pitch 1000 Å                                                                       height 8000-13000 Å                        11e  stainless steel                                                                        A1 (1500 Å)                                                                       SnO.sub.2 (1500 Å)                                                                 ZnO (25000 Å)                                                                         8.5%                                    plate (SUS 430)                                                                        irregularity                                                                          irregularity                                                                           pitch 17000-26000 Å                                      pitch 1000 Å                                                                      pitch 1000 Å                                                                       height 10000-16000 Å                       __________________________________________________________________________

                  TABLE 9                                                         ______________________________________                                        Sample                                                                              Subst. temp.                                                                             Appearance SEM Observation                                   ______________________________________                                        13a   room temp. lustrous   irregularity of a pitch of                                                    ca. 1000 Å                                    13b   100° C.                                                                           slightly turbid                                                                          irregularity of a pitch of                                                    ca. 2000 Å                                    13c   200° C.                                                                           turbid     irregularity of a pitch of                                                    4000-7000 Å                                   13d   300° C.                                                                           turbid     irregularity of a pitch of                                                    4000-10000 Å                                  ______________________________________                                    

                  TABLE 10                                                        ______________________________________                                        Sample                                                                              Subst. temp.                                                                             Appearance SEM Observation                                   ______________________________________                                        14a   room temp. slightly   irregularity of a pitch of                                         yellowish, ca. 1000 Å                                                     lustrous                                                     14b   100° C.                                                                           transparent,                                                                             irregularity of a pitch of                                         lustrous   ca. 1500 Å                                    14c   200° C.                                                                           turbid     irregularity of a pitch of                                                    3000-7000 Å                                   14d   300° C.                                                                           turbid     irregularity of a pitch of                                                    4000-9000 Å                                   ______________________________________                                    

                  TABLE 11                                                        ______________________________________                                        Sample                                                                              Subst. temp.                                                                             Appearance SEM Observation                                   ______________________________________                                        15a   room temp. slightly   irregularity of a pitch of                                         yellowish, ca. 1000 Å                                                     lustrous                                                     15b   100° C.                                                                           transparent,                                                                             irregularity of a pitch of                                         lustrous   ca. 1500 Å                                    15c   200° C.                                                                           turbid     irregularity of a pitch of                                                    3000-7000 Å and a height                                                  of 1500-3000 Å                                15d   300° C.                                                                           turbid     irregularity of a pitch of                                                    4000-9000 Å and a height                                                  of 1500-4000 Å                                15e   200° C.,                                                                          turbid     pitch 6000-10000 Å                                  immersed in           height 3000-8000 Å                                  10% acetic                                                                    acid soln.                                                                    for 1 min.                                                              15f   200° C.,                                                                          turbid     pitch 9000-14000 Å                                  immersed in           height 5000-10000 Å                                 10% acetic                                                                    acid soln.                                                                    for 1.5 min.                                                            ______________________________________                                    

                  TABLE 12                                                        ______________________________________                                        Sample                                                                              Subst. temp.                                                                             Appearance SEM Observation                                   ______________________________________                                        16a   room temp. slightly   irregularity of a pitch of                                         yellowish, ca. 1000 Å                                                     lustrous                                                     16b   100° C.                                                                           transparent,                                                                             irregularity of a pitch of                                         lustrous   ca. 1500 Å                                    16c   200° C.                                                                           turbid     irregularity of a pitch of                                                    3000-7000 Å and a height                                                  of 1500-3000 Å                                16d   300° C.                                                                           turbid     irregularity of a pitch of                                                    4000-9000 Å and a height                                                  of 2000-4000 Å                                ______________________________________                                    

The rear reflective layer of the present invention increases lightreflectance and achieves effective trapping of light in thesemiconductor, whereby light absorption therein is increased and aphotovoltaic device of a high conversion efficiency can be obtained.Also, the reliability of the photovoltaic device is improved since thediffusion of metal atoms into the semiconductor film is suppressed, theleakage current is suppressed by a suitable electrical resistance evenin the presence of local short-circuits in the semiconductor, and sincea higher chemical resistance reduces the danger of new defect formationin the subsequent process.

Also, even if the metal layer becomes exposed in the formation of theirregular surface structure which is required for realizing theeffective optical trapping effect, the diffusion of metal atoms from thethus-exposed area to the semiconductor layer can be prevented by theformation of a second layer. Furthermore, the frequency of leakagecurrent between the thus-exposed area and the upper electrode throughlocal short-circuits in the semiconductor can be extremely reduced bythe formation of said second layer, whereby the reliability of thephotovoltaic device can be improved.

Furthermore, such rear reflective layer can be produced as a part of amass production method, such as a roll-to-roll method. As explainedabove, the present invention contributes greatly to the practicalapplications of photovoltaic devices.

What is claimed is:
 1. A method for producing a photovoltaic deviceprovided with a metal layer with a smooth surface, a transparent layercomposed of a plurality of layers on said metal layer, and aphotovoltaic layer on said transparent layer, which comprises utilizingthe difference in forming temperatures of said plural layers of saidtransparent layer and forming an irregular surface structure in a firstsaid layer in contact with the smooth surface of said metal layer, andsatisfying a relation T1>Tn between the forming temperature of T1 ofsaid first layer the forming temperature Tn of the second to the nthlayers, wherein said layer forming temperatures are numbered as thefirst layer forming temperature T1, the second layer forming temperatureT2 . . . Tn from the metal layer side.
 2. A method for producing aphotovoltaic device according to claim 1, wherein the irregular surfacestructure is also formed in said transparent layer by depositing a layerin which said irregularity is to be formed and then immersing thesurface of said layer in an aqueous solution of an acid, an alkali, or asalt.
 3. A method according to claim 2, wherein said acid is aceticacid, sulfuric acid, hydrochloric acid, nitric acid or perchloric acid,said alkali is sodium hydroxide, potassium hydroxide or aluminumhydroxide, and said salt is ferric chloride or aluminum chloride.
 4. Amethod for producing a photovoltaic device provided with a metal layerhaving a smooth surface, a transparent layer composed of a plurality oflayers on said metal layer, and a photovoltaic layer on said transparentlayer, which comprises utilizing the difference in forming temperatureof said plural layers of said transparent layer and forming an irregularsurface structure in at least one layer other than the one in contactwith the smooth surface of said metal layer, and satisfying a relationT1<Tn between the forming temperature T1 of said first layer and theforming temperature Tn of the second to nth layers, wherein said layerforming temperatures are numbered as the first layer forming temperatureT1, the second layer forming temperature T2 . . . Tn from the metallayer side.
 5. A method for producing a photovoltaic device according toclaim 4, wherein the irregular-surface structure is also formed in saidtransparent layer by depositing a layer in which said irregularity is tobe formed and then immersing the surface of said layer in an aqueoussolution of an acid, an alkali, or a salt.